When Apple bought Beats I was about to kill myself, because Apple said they bought Beats, because they love music.... Well why not buy the best headphone-making-company then?? Sennheiser-headphones has on of the most clear sounds I've ever heard, and Beats is just... bvaaadar....
I was also originally baffled by their decision.
I imagine they bought Beats for the execs, such as Jimmy Iovine etc, who have large clout it the music industry. Further, the team at Beats were excellent at making an inferior product (compared to Sennheiser, as you mention) into something extremely popular and chic. Apple would benefit from this not only in their own advertising campaigns (which they're currently fighting an uphill battle against Samsung), but also for any future accessories such as their purported iWatch.
Also, the team behind Beats Music, which has received good reviews as a curated music service. They need the help for iTunes Radio.
Apple mentioned they didn't plan to rebrand Beats headphones as their own (thank goodness), so if anything, Beats headphones might actually increase in quality after Apple's meddling.
So, it seems Apple wasn't after Beats' hardware at all, but instead their creative team. Apple is best at inventing new product categories and making them insanely popular -- the copiers gain an edge on them in a matter of years (though Apple still maintains their edge through their devotees and high profit margins). By buying Beats' creative team and advertising capability, it should let them stay at the top of their game.
Or something like that.