Well, finding and connecting Wifi can be tricky, and usually fails when I would most need it. Also, there's no roaming, you cannot just walk around and have the data flowing while the device jumps from one network to another (at least without some serious VPN hackery).

I actually had a N810, and found it not so useful just for the reason it didn't have a cell radio. While Wifi is more practical than a wire, it still has wire-like restrictions on one's mobility.

I admit that a Wifi-only N900 might be attractive to some if that meant it was a lot cheaper. Just not for me.