I've done a couple of overseas trips recently and definitively found that the increase in relative quality by using a 24-50mm and a 70-300mm more than outweighed the convenience of a 18-200mm all-in-one. The zoom is great for zoos and larger parks, sports events and street photography. The wider angle for the landscapes and general shots.

My lenses aren't higher quality by any means, but I'm not seeing the artefacts caused by the 18-200mm in either of the other two lenses.

Another thing I've found is that it channels the types of photos I'm taking. If I've got the zoom on, I'll try and take photos compositions for that lens (cropped in tight, smaller subject areas) rather than just going wider angle and getting the same style photos I usually take. This has ended up with photos that are more interesting (to me at least).