It's not a deal breaker for me, I understand that Apple puts a lot of restrictions in place for security reasons, and for good reason. But this is something that I've anticipated coming for a long time, then got excited when I stumbled across it, then frustrated when I realised iOS is the only platform it's not working on yet...I'm sure if Android, Blackberry and Windows Phone can get it working, they can't all be insecure.
The most likely reason that Apple don't allow is that doing so would result in a huge number of apps exploiting the ability for what most will deem unreasonable purposes, which does happen and has happened on Android.
I've not heard of
telegram. Is that a competitor to What's App? Or does it have it's own spin on messaging?
Ignore the advertising blurb, as quite frankly security on all of these services is as good/bad as each other, i.e. don't presume. There are services explicitly designed for that.
Telegram became well known and reached the top of the App Store the week that WhatsApp was scooped up by Facebook. From a user perspective it provides great clients on all common platforms (OS X, iOS, Windows, WP, Android, Linux and web), is darn reliable, fast, the ticks make sense (user-received/user-read, WhatsApp is server-received/user-received, which is completely illogical, we only care if the other user has received the message!), it offers both compressed and uncompressed sharing, list goes on really.
Not trying to advertise for them however I've been through all the major services and Telegram is the only one that has kept me. It's essentially a more polished version of all the chat services you are already familiar with. I can't really find any drawbacks in terms of usability. Only potential issue is the user-base being smaller (but still more sizeable than that of imo.im and whatever else - chances are you'll run into
someone using Telegram), however I've found that the people that try it stick with it/move completely to it.
Depends what you're looking for really. Quite frankly, if all you're looking for is something that is multiplatform and better than iMessage, a carrier pigeon would be faster and more reliable.