I think I understand how M&A teams at places like Yahoo! work. It's all about what's "hot" with maybe some fanciful vision of integration into Yahoo! in a way that will never happen. The M&A team is drinking the same koolaid they're selling to top level execs, founders and VC's of companies they're looking at, bringing a gigantic checkbook with them.
Zimbra was actually a good kind of acquisition, because they did a good job integrating it with Yahoo! Mail right away, and what you see today is derivative of that purchase. It made a lot of sense for Yahoo! to buy a fancy webmail interface since most people using Yahoo! use it for their email. Some other good examples of acquisitions are Google buying Android and Apple buying the chip manufacturer that led to the A4 for the iPad. Bad acquisitions are when it seems like it's about the buzz and it's hard to visualize how the purchase can help the bigger company. Those acquisitions go the way of Dodgeball and Jaiku.
Foursquare is a mobile company and Yahoo!'s (lousy) presence on mobile phones is restricted to un-installable bundled software deals like with AT&T on the Backflip. Yahoo! is not a mobile company, it's not the first, second, or even in the top ten or possibly not even in the top fifty companies you'll think of when someone asks you to name companies that are movers in the mobile space.
Foursquare isn't going to do well at Yahoo! because it doesn't make sense.
I usually don't like Arrington's rants, but this one was inspired.