Quality differences are definitely going to be apparent, depending on the situation; I shoot weddings w/ a max of 2 pro-sumer cameras, often only 1, and fill out the other 2 - 3 cameras w/ consumer models. The big noticeable problems are color and resolution. My Panasonic PV-GS320 rates pretty well in color next to my AG-DVC30, as long as I'm shooting in well-lit environments. (Like outdoors.) However, unless I'm in close to the subjects the resolution on the GS320 drops considerably, especially if zooming in to achieve closeups. The reason for this is two-fold, since the CCDs in the consumer camera are smaller and there's less glass (read: smaller lens) to shoot through.
So the weaker camera can work, you just need to use it in well-lit and balanced environments moved in as tight as you can manage and keep it zoomed out so its utilizing as much of the glass as possible.
There are a variety of places utilizing this can be helpful; in an upcoming shoot this fall I've got a frantic chase scene I'll be shooting and I plan on fixing my consumer camera to the bottom of my steadicam to shoot foot- and leg-level shots while also shooting face and body shots with my DVC30 on top. Shooting both of these at the same time helps with speeding the shoot up and easing the matching and mismatching of movments and audio. The nature of the scene being so fast-paced, the lower resolution coming off the GS320 will be barely, if at all, noticeable.
In the end my biggest recommendation is to try shooting both cameras simultaneously in a variety of environments and decide for yourself when you can and when you can't utilize both.
Hope this helps.