Actually, it is the people who are into playing that can be just as picky if not even more than arrangers. During arranging, you can blend the sound as a whole and so sometimes not-so-good sounding pianos can be OK with better sounding modules.
When playing just the keyboard (usually piano and keyboard sounds when just playing), bad sounds become more evident and obvious. Again, when playing with a band, that again blends in. But I have not seen any keyboardist playing in a band using Casio - I'm sure there are though.
Anyway, sound is only a part of keyboards. Touch is just as important. For the Casio, the touch just doesn't cut it. However, I've tinkled a little at the weighted Celviano - it doesn't seem "terrible". But I've only tinkled for a few seconds, so further comments will be unfair. One of my friends has one - maybe I should pop by his place one day to try out for 30 minutes and see.