Pretty much the only obvious use for a UV filter on digital is as a lens protector. The only non-UV lens protectors I've seen have been uncoated glass (which I'd steer clear of due to potential reflections), though it may be you can get coated ones too. I see no advantage / disadvantage to using either, so long as they're good quality from a reputable brand, and multi-coated.
I went through a fairly lengthy phase of putting UV filters on every bit of good, modern and classic glass that I own (I dread to think what I spent on filters over a two or three year period
). Although most of my photos turned out just fine, a not insignificant number were affected to a greater or lesser extent by either reflective flare with bright light sources, or weird "stripe" artefacts (the latter only seemed to appear at longer focal lengths). In some cases, these issues were only noticeable at larger reproductions; in others, they were more obvious. Some were repairable (though it was a pain and time-consuming to do so), but numerous otherwise-good shots were fit only for the Recycle Bin
At that point, I changed my approach. I left the UV filters on my lenses while not in use. Then, when I'd start shooting, if conditions allowed I'd simply remove the filter. The thing is, conditions were fine 90% of the time. So, now, I've removed all the UV filters from my lenses, and simply carry what I need with me. If conditions are rough, I might fit them before I leave home or get out of the car. I will say, my own preference is to shoot with a UV filter fitted in very wet / windy / sandy / gritty conditions, as I'm happier cleaning the front of a filter rather than the front lens element... and I abuse my UV filters, occasionally cleaning them in the field with tissue papers, rags, my shirt tail etc.
On the subject of stacking... I just wouldn't do it. Even with multi-coated filters, the chances of reflective flare are increased way too much for my liking; and, whilst you may not have noticed any significant differences in your images when stacking filters, when you eventually
do - and I'm confident it's going to be
when, rather than
if - you may find yourself throwing otherwise-good photos away. From experience, I can say that's awfully frustrating