Everybody that I know or meet uses iMessage. Why would you need to download another app just to talk when you got one built in?
You don't. Some other apps are better than the pre-installed ones though. For example, WhatsApp lets you send photos etc to everyone over data, rather than using MMS messages for people who aren't on iPhones. It also has benefits like better group chat management and web/desktop clients, you can also message up to 256 people at a time if that's your thing.
I mean, my laptop came with IE installed, but I downloaded Chrome because I like the extra features. Obviously I don't have to get other people to download it to talk to them on the internet, but it's an example of wanting to use a different program to accomplish built in tasks in a better way.
I don't think your perfect record of only meeting people with iMessage is going to continue as it's a staggering coincidence, but if you don't want to talk to anyone on anything else then go right ahead and keep using iMessage, you'll probably be fine.