I think the biggest thing you're struggling with is trying to justify it in your head as a need. Do you need a cell phone? No, but they make life easier. Maybe it's because you're used to your BlackBerry, but if you had an android device you might realize how much better and easier it is than a 9700. Also, in terms of typing, physical keyboards are great, but once you get used to the touch screen, it's just as fast. In fact, I now have trouble typing on my 9700 since I'm more used to Swype.
As far as I know, there are no apps out that are as good as bbm, but there are several alternatives that are close. I don't personally use any, but it might not hurt to do a Google search and try a few out. And if you do, try them on your Nexus 7 too since an app that's not so great on BlackBerry might be really great on Android.
That said, I think if you did get the Nexus 4 you would soon realize that there are a lot more reasons to switch than 3.5g and a Wi-Fi hotspot. It's great to not have to carry your tablet everywhere. Google maps is amazing, even if you do have GPS in your car, when it can't locate something, you need to check real-time traffic or you're in a friend's car, it can be invaluable. Then there's the browser, which is night and day better than BlackBerry and the extra screen space really helps too. Also, you can get more apps that are cheaper and way better and allow you to do just about everything you want to with your phone. And having Google back up your calendar and contacts over the air automatically can be a real lifesaver sometimes- not to mention, your calendar will be on both devices. There are plenty more reasons that aren't coming to me off the top of my head, but they easily make up for losing a physical keyboard and bbm. Plus it will be faster to go between two devices that work the same and you can always take advantage of the return policy if you're just too attached to your BlackBerry.