At hiring time, it can be very hard to tell the difference between "vanity/primadonna devs" and "devs who really like certain technologies which will possibly b

At hiring time, it can be very hard to tell the difference between "vanity/primadonna devs" and "devs who really like certain technologies which will possibly blind them". And even among the latter group, there are tradeoffs in hiring them. There are many reasonable places to set the dial on how much a hiring manager wants to take risks on that. But I maintain that, as with many other hiring criteria, erring on the side of caution is a reasonable choice. As I said, I think being excited about a cool language is a mixed signal.