The main difference is that iMessages count towards iCloud quota, whereas (most?) other messaging services have free storage.

iMessage doesn't require you to store history in icloud, it can just store everything locally if you want. But yes, I'd rather not have stored history, or the option to pay for storage than to have all media crushed beyond recognition.

A few times I've wanted to print something and found it was sent over an IM app and compressed to 100kb rendering it useless.