That's the showstopper right there. I might want to use IM more than anything, but as long as the "people I communicate with" is not limited to my geek buddies with smartphones or spread through half a dozen different IM protocols, I won't be able to do that
Also, not sure if it's an European thing, but SMS reception is free (in fact, I don't know about SMS-less plans). As for sending, the #@%#$ providers here reintroduced per minute billing, so for short reminders/confirmations it's actually cheaper/less bothersome to use SMS than do a voice call (60$ ? Even on the worst plan here that's several thousand messages !).