Here is my point of view on this (as G-MANN wanted it :D)

Of course it is always good to have better pictures.

But I think it is more important to list movies without pictures before replacing existing pictures by better ones.
So if you want to watch the movie anyway, why not replace captures. But if you watch it just for that it is probably not very useful. When pictures are 640-wide they are often already rather good (sometimes I do not even bother replacing them even if I could).
Of course if the movie has bad quality picture it is worth "upgrading" them.

And it is also important to not delete the background ones that you do not like

Sometimes the difference in quality is hard to notice.
Recently I watched http://www.imcdb.org/movie_814022-Bangkok-Dangerous.html
There were already pictures so at first I thought I would not do captures, but the ones on the site were really not very good (and less than 640 pixels wide I think). Those that I uploaded were not from a DVD but I doubt that pictures from a DVD would make much difference in this case (colors may be a little more bright, but the current pictures are already rather good for sharpness & size I think).

And now with HD movies, any DVD pictures could be replaced by better ones anyway...