Good idea, at least down to about 30 degrees. (That's as low as I've ever taken one; for all I know, you might be able to use them even lower.)
I tried the Thermarest Ventra, and liked it a great deal. However, it was half a pound heavier than my WM Mitylite, and both were rated as 40 degree bags. The Mitylite is actually a hoodless, semi-rectangular sleeping bag that can be zipped fully open to form a quilt; I find that I usually end up using it that way. Add some insulating clothing (in my case, down pants, hooded down jacket, down mittens, and down booties) and I can take it to 30 degrees. I never tried it with the Ventra.
I only took the Mitylite to 30 one time, as an experiment. When the temperatures are predicted to be lower than 40, I usually just take my Megalite.