A single tech stack can only teach me a limited amount. When I build something around an unknown-to-me language/framework/tool over a weekend, I usually walk away with new insights.
For example, playing with Meteor (js) was the first time I was able to understand the concept of pub/sub.
That knowledge can sometimes be brought back to other tools I use, sometimes not. When it cannot easily transfer, the new tool gains brownie points. Playing with the "flavor of the month" taught me something useful. If I stayed with the same established workflow I always use, then I wouldn't have had a reason to encounter pub/sub.
That's a very bad thing to spread. Haskell might be deep, but after using it for a whole year in production, any developer worth his salt would know enough of it to build non trivial things.
This type of comments unfortunately keep developers away from Haskell. Haskell is no C++, you can actually learn it, and it is a very good language.
I have to wonder how true this is. Someone who gets good at whatever their domain is is probably better at that domain than the leapfroggers.