BBM does a lot more than just text. On the chat side, you can send pics and voice messages. The confirmations come in very handy. My wife works at a hospital. I would send her a BBM and I could see when she read the message. It might take a while if she was with a patient. Plus, you can set your status to let people know you are busy, unavailable and so on.
Add to the texting side, there is BBM Groups. This made it very easy to link up with people who have similar interests, share photos with the group, and other information.
We actually had a BBM Group for our family. The group calendar let us add family events to the calendar that everyone would need to know about. School events, works events, and so on. One person adds it, it shows up on everyone's BlackBerry.
We did the same with a list for groceries. Anyone could add to the list and the items would show up on everyone's BB. Whoever went to the grocery store would check things off and it would update for everyone.
Now that we moved over to Android, we use Google calendars, GroceryIQ, and LiveProfile/Kik to pretty much replace BBM. I haven't found a good replacement yet for group chats, but there are different apps that can do some group stuff.
I think the big thing is that with any platform, you have to have the same app installed on everyone's device. Sure, there is GT and SMS, but neither of those have the confirmations. So you have to use something like Kik (not allowed on BB), LiveProfile (should be on BB later this week), PingChat, or WhatsApp ($) to get similar BBM chat experience. Plus, if you want some of the other features, like calendar and list sharing, you have another set of apps to make sure everyone has installed.
BBM was just convenient on BB since it was already installed, and had all those things integrated already. Even if it is chat-only, BBM on Android would probably be a welcome addition.