Honestly, I can't think of any drawbacks to using the setup you have now if you're happy with the sound and it does what you need. If the speakers will fill the space, I don't see any benefits to using a dedicated guitar amp at all.
I'm also happy with the sounds I'm getting from my setup at home which consists of a V-Amp Pro, Auzentech HDA X-Plosion sound card, Musical Fidelity X-A2 amp and a pair of B&W speakers. The V-Amp Pro really took me by surprise when I got it because I wasn't expecting it to sound all that good, but boy was I wrong, this cheapo piece of gear is just great, and it is way more versatile than any amp I have ever owned. I think I would be confident playing a large venue with it plugged into a PA, or a smaller venue just using the stereo speakers.
Again, if your speakers are decent enough I don't see a problem. Bear in mind that the size of the room will make a huge difference, plus when people are in there a lot of sound and volume will be soaked up, so it'll probably be very different to how it sounds at home. If you think the speakers have enough power to handle a small venue though, go for it! I'd love to know how it goes, btw.
I used to be a complete
tube amp nut, I wouldn't use anything but my Marshall's, but nowadays modelling amps are very good, I wouldn't hesitate to use one in a live setting.
