Imagine a hiring situation. There's two candidates both with a few years of experience. In the blue corner we have someone with good broad design skills in the style of design that you favor (in my case that would be things like DRY, judicious use of patterns, TDD, communicative code etc, but the actual list isn't important - just that it's what you favor). However she knows nothing of the particular platform technology that you're using. In the red corner we have someone who has little knowledge (or interest) in those issues, but knows your platform really well - edge cases in the language, what libraries are available, fingers move naturally over the tools. Assume all else about them is equal (which it never is except for thought experiments like this) and that your team doesn't have any gaping holes that this candidate might fill. Which one would you prefer?
My answer is simple, I'd take the one in the with broad design skills. I've always held the view that a good programmer should be able to pick up a new platform relatively quickly. Learning basic design aesthetics is both harder and carries over better to new platforms. Good design practices that matter in Java are equally valuable in .NET. Not being familiar with the platform does slow you down (how do I get a literal class name in C# again?), but producing well designed code is what really makes a difference.
Broad design skills aren't completely portable. Java and .NET are mostly equivalent as languages - moving to Ruby, however, changes more. Moving to a significantly different beast, like functional languages, is a bigger shift. In any case, you can't just blindly replicate all design habits in a new environment. But if you're aware of the new world, an awful lot does carry over.
We've seen this principle prove itself at ThoughtWorks. In our early days with Java, we found the skills experienced developers had learned in Forte gave us excellent instincts for working with Java. We moved away early from the EJB-dominant thinking, and I think it was experience with other platforms that guided us. We saw it even more strongly with .NET. Time and again we saw that good developers with a Java background were rapidly more effective than those with a longer .NET or Microsoft background who lacked those skills. The difference was visible in weeks, not months (and sometimes days).
At the moment we see this shift most notably in Ruby. We've had quite the run of Ruby projects this year, and often we turn to people with long experience in curly-brace languages to fill the need. Again we've seen the value that broad design skills gives us.
It's not always a sure thing. I have seen cases where someone experienced in another platform just doesn't desire to get in and learn the new one. Desire to learn is a necessary component here - I'd take the single platform specialist if he wanted to learn broad design and the broad designer didn't want to learn the new platform. It's also essential to have someone on the team who knows the platform well.
I'd say most people at ThoughtWorks prefer design skills over platform knowledge. Many clients don't share that point of view - which can lead to some difficult pragmatic and ethical choices.
What happens if you have someone you want to bring onto the team with strong design skills and no platform background - yet the client insists on at least two years experience on the platform. In your professional judgment, the broad candidate is going to be more productive than anyone else available. You need to be honest with your client, but on the other hand he is paying you for your professional judgment. Does this change if the client has given you responsibility for delivery of the project?
For us these questions are more charged because there is a financial interest involved. If we add a ThoughtWorker to a team, then we bill for that person. If a client hires a platform specialist contractor, we don't get that income. For many people this is a crucial fact in the situation, yet I expect our project managers are wise enough to know that the risk of adding the wrong person is much more important than one billable income.
Consider another case where you're open with the client and the client demands a reduced rate for the broad design person due to her lack of platform knowledge as she'll be learning on the job. You're sure that she, despite that lack, will be more productive than the competing platform expert due to those design skills. Should you accept a reduced rate?
It's the nature of our, and most other, professions that you learn on the job. A platform specialist also has to learn broad design skills if he's going to produce maintainable code. Here it's important to remember that not just is it usually harder to learn design than platforms, it's also less certain. Given a motivated broad-designer, I can be pretty sure she'll pick up a platform in time. But there's no guarantee the other way around. Some people are good at learning details of a platform, but never figure out how to write clear code.
I've talked here about broad design skills - and I do believe this makes a difference on the technical axis. But there are other dimensions of broadness too. Most risk in software projects lies in the communication between businesspeople and programmers, so a candidate who can communicate well with users brings a great deal to a team.
A similar issue is knowledge of the problem domain. Often clients want people who already know their business, yet are surprised when someone rapidly gains enough understanding to be useful. I've long held that it's the ability to collaborate with others which is central here. By collaborating with a domain expert, or a platform expert, someone with broad skills can be become effective very quickly. Knowledge of other domains often introduces surprising insights into a project and similarities often crop up in sup-rising places. It's remarkable how often things like core accounting patterns crop up in places that don't look like accounting on the surface. In the end it's the ability to work with others, coupled with being a fast learner, that is the critical skill.
I'm not dismissing deep platform knowledge here. In an ideal world every team member would be excellent broad programmers with several years platform experience, good familiarity with the problem domain, and written similar systems at least twice before. But we all know how far our world is from that ideal. You need some platform knowledge on a team, and if it were a gap I would reach for the platform specialist to fill it. But that doesn't alter my default position to prefer broad design skills most of the time.