When you Upa and Roll someone you trap an arm and the same side leg so that they cannot post on them when you roll to the side. If you try to Upa and Roll without the leg trapped, they can stop the roll by posting their foot or knee. However, when you grapevine the legs from the top, you are trapping your own legs for the bottom man.
I am not saying grapevining the legs is a bad idea. It is definitely useful, but you become a bit vulnerable to the Roll.