Simple answer really, Most projectors run films off reels still (I'm no longer in the cinema biz but I think that's right) some newer cinemas may use digital projectors which would improve quality.
The thing with the older style projectors that are most common is that the film stretches and can become damaged over time.
It's a standard practice to previw a film with a couple of cinema staff or just projectionists a few days before it's shown to the public to ensure that the reel is of good quality and there are no major problems.
You may notice that whilst you are watching a film you see white spots or "glitches" in the picture this is just were the film has been run a number of times, which is why yo umay notice it more on a film that has been out longer than one that has been ou a day.
hopefully the majority of cinemas will update to digital projectors however I think it'l be a slow process as the distributors will need to phase out the older film 'reel' and introduce the new disk version (again I don't work in the cinema biz but this if what I ran across whilst I did)
this leaRAB to the obvious, my dvd copy is better, which is quite true as unlike tapes DVD won't get worn with time, except if it's scratched. if you are old enough to remember tapes, you'll remember that if you watched a tape enough it would become unwatchable due to static covering the majority of the picture, which was caused by the tape being stretched from numerous viewings and rewindings.
anyway I still believe going to the cinema is the best and most enjoyable way of seeing the latest and greatest films, and I would have to say depending on th age of the cinema you visist the picture quality shouldn't be so dreadful that it detracts from the film.
Hope that helped!