If you don't have a texting plan and someone sends you a text, it costs you money. Unless you know to have your carrier disable it on your account. iMessage doesn't necessarily cost you anything, except for tiny bits of data. Chances are this isn't going to be enough to put anyone over their data usage.
But even so, you can turn it off. Just like you could turn off texting. It's really not any different. If you don't know to turn it off, it could cost you money. If you're worried about your friends sending you video, tell them not to. I can do that just as easily as I used to tell people not to send me a text unless it was an emergency because I would get charged for it (back when I didn't have a texting plan at all).
iMessage has the ability to save people money. Many won't need an unlimited texting plan anymore. And if it was turned off by default, everyone here in the forums would be screaming that it didn't work, etc. etc.
Well said. Count me a one of those that iMessage saves money. I pay $5 for 200 text messages on AT&T and I regularly now exceed that amount. The only other text message plan available to me is unlimited for $20/mo but as a significant number of my messages are sent to other iPhone users, I now no longer need to upgrade, thus saving me $15.
And while I don't know the specific amount of data used by an iMessage, I can't imagine it's very much and doubt these messages significantly contribute to your data usage, unless you're sending thousands of messages. I understand the OP's MMS group message scenario but really, how often of you need to send these messages? In these cases, shut off iMessage and use your allotted text plan. Also, aren't photos sent via iMessage or SMS significantly compressed to reduce file size?