It depends what else the phones are being used for. If they are regularly pulling down audio data for the web, or even fat, non-mobile-ized web pages, on a regular basis, then the incremental bump from a few dozen spam emails a day might be hard to notice...
But if people install no vetted application then they will get unvetted results.
Now if only the goverments realy stamped hard on SPAM, maybe if the music industry had a vested interest then it would soon get stamped out. Its a fine revenue that goverments could tap into and the public realy wont be complaining for once. Please goverments - go get hard on SPAM and levy huge fines and make us all happier.