you were deep in OF, so boesh was only getting bench time. He would not get regular time unless cruz and hamilton are hurt. You have good pitching, and cahill really is only your 6th-7th best option.
I am on the fense, i think that if things fall apart and you need to sell you will get more for cahill, so he is safer. Your OF now has no need for him, and rasmus has more pedigree (former baseball america top 10 prospect). Boesh had a bad 2nd half last year but has shown he can be a top 100 player.
I do not object to the trade on the surface, i just don't see the reason why you would bother. you need a closer now more, and an upgrade at catcher long term more.