The reasons are repeated often in the same discussion. In short.
1. Forget 3D games.
2. Takes a toll on CPU performance.
3. Eats some main RAM.
4. No upgrade possible in such systems.
In closing, such systems still can beat a machine from 2 years ago.
Bob
"And, yes, as Bob through in as a curve ball, try to avoid notebooks that have shared or integrated video
memory."
I have read this before, in both the desktop and notebook forums. Could someone explain this pitfall to me a little more. I usually go through Dell, and, since I'm, in the market for a new pc, I've noticed that during configuration, I come across things like 512 SDD shared ram and stuff like that; is this what you're speaking of? Why is this bad? Any retort would be much obliged.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic