I thought this video was interesting:
Brain science also talks about focusing on strengths. If you're good at X and bad at Y, you might end up spending a whole lot of time on Y. In addition to making you miserable, it's also just not that effective. While at first glance you might think if you're bad at something, then it's easier to improve because there's so much possible improvement you could do, it's usually more like the opposite. If you're good at something, your neural pathways (as in, the physical structures of your brain) are better set up to do it, and you have more ability to learn and grow quickly at that thing. So if possible, develop your strengths. And find other people whose strengths are in your areas of weakness.
In a small company, we don't usually have this luxury though. Probably the lead designer of Portal 2 didn't do all the graphic design for the box it comes in, because Valve probably has a team devoted entirely to graphic design for marketing materials and packaging. (Meanwhile, I do both things.) That said, strengths-focus is definitely something even a small company should think about when adding any employees, or even contractors or volunteers. Just a little help in your areas of weakness, you could then have much more time to develop and use your strengths to an even better degree.
In other words, Valve's handbook's stance on that makes sense to me.