The Intel GPUs suffer serious performance issues, in part because they have no dedicated memory of their own (they just use the same system memory you're trying to use for everything else). That's fine for day to day use and even basic utility like playing videos, and may save power too, but if you want to actually make the GPU do serious work it's not a solution. Sometimes the onboard GPU is actually a hindrance when it won't get out of the way and let the real GPU do the work.