Only two reasons I can imagine/guess Apple would want Beats:

  1. Jimmy Iovine & Dre
  2. The subscription deals Iovine did with the labels

If Iovine's deal/s allow for multiple users per account, e.g. a "family account" type arrangement as seen in the AT&T deal, Spotify is toast.

Of course it may be that Apple is happy to own the Beats brand, to continue to build a great non-Apple branded headphone biz. But it's still about point #2 as Apple aims to build a streaming music service.

Update: Walt Mossberg agrees.

Update, 2017: Spotify is clearly anything but "toast". Apple has not executed Apple Music well while Spotify continues to add value.