The dedicated graphics solutions found on mobile CPUs are already underwhelming relative to their desktop counterparts. Heck, the mobile CPU itself is already underwhelming vs. it's desktop big brothers. As others have said, the power consumption levels will be lower, and their heat levels will be higher in the small enclosure.
Not being able to draw as much power, heating up faster, and lower performance vs. desktop counterparts (which is often the result of the first two), means that to play any modern 3D games, even on the lowest settings, would call for a dedicated mobile GPU... even one that's not powerful.
That being said, if all you're playing is, say, World of Goo, or Bastion, or Osmos, or And Yet it Moves. If your 'casual' games are games on that caliber, then I think the build-in graphics on the processor would be OK, pending the processor is a fairly modern one.