Yeah, the Leslie does rock!
You definitely need a little more than soundblaster to really enjoy the VST software, unless you were able to get decent latency from just that. But depending on what you're trying to do, some things will be better with the computer (for now at least) and some things will be worse. In my case, I'm just trying to play regular sounds (piano, organ, rhodes) live with a better sound, without carrying multiple keyboards around. But it's really for the sounds. I'm not recording with the laptop nor do I need lots of effects like guitar players. So, all I really need to run is a VST host with a couple of plugins.
Guitar players, on the other hand, need a whole slew of effects and stuff, and not only that, they need to be able to control in with foot switches and what not. So it takes a lot more tweaking and setting up, and a whole bunch of other stuff just to get a decent setup. And there's the possibility that it doesn't even sound as good through the computer as the real thing (like you mentioned). Besides, all the guitar stuff is already portable, so you don't have as much incentive as keyboard players to use computer software to achieve a certain sound.
That being said, let me also say I got a Peavey 1600x midi controller, which is the greatest thing since sliced bread. They don't make it anymore for some reason, but this thing is awesome. It's a programmable midi controller with 16 knobs and 16 buttons, and it's built very well. I use it to adjust anything in my VST software, so it almost feels like I'm not even using the computer. I have to say, before I had the 1600x, using VST software just wasn't that practical.