So do you use you ski jacket in the summer time? No, you use different jackets and/or you layer.
I know of a few people that use various combinations of suits for different conditions. I have different wetsuits for different conditions. To each his/her own.
Just a couple little things to consider: If you are thinking about getting a new suit to go over something you already have think about:
1) Make sure that it is large enough to fit comfortably over the other suit. It may then be too big to use by itself. Just think it through carefully.
2) If your outer component has a zipper in the front, it is easier to regulate water flow if you get too warm. If you get components to work in the coldest environments that you will typically dive, and then try to use the same (or variations on the same) pieces for warmer water, it can be nice to be able to open the zipper part way and allow some water flow. That is much harder to do with a rear zipper.
Just some points to ponder.
FYI: I use a 2-piece 7mm wetsuit in Monterey / Lake Tahoe, 1-piece 3mm full suit in 65 - 73 degree water and a .5mm (1/2mm) full suit when the temps are above 73 degrees. The Drysuit is still an expense that will have to wait. Ever diver is different.
Wristshot