I think it will really depend on what you want to take pics of.
If you plan your day, then having an 18-55 and a 55-200 in your arsenal is a good option as you only then need to take one or the other with you, and maybe a prime too.
However, having a super lens like the 18-200 on holiday etc is beneficial too - only 1 lens to take and look after and worry about, and as it's more than likely attached to your camera body on a strap around your neck, then chances are it won't go walkies without your knowledge
The disadvantage is however, it's a heavy lens, ~1/2kg and you will be lugging that around your neck all day (or whatever part of it you have the camera in use).
The argument to that would be 2 lenses might weigh the same too, but you have a choice then as to which to use.
Another thing I noticed when I hired a 18-200 a while back is lens creep - this particular lens seems to do a fair bit of it
Finally, any damage to your 18-200 is costlier then say to one of your 18-55 or 55-200 lenses. I'd certainly be crying if my £500 lens went belly up, but might sob a little less if it was my £90 18-55