P gives you plenty of rope to hang yourself with, □ does not.
For example, in P, you can have exposure compensation set to +/-3. If you forget that you did that, and don't notice in the viewfinder, when switching back to P, your shots will be exposed improperly.
Since I've started shooting in manual more, I really don't use the other modes much at all. I'll use full auto at times when I don't want (or have time) to think about my manual settings. This week-end's barbecue, for example was on a sunny day in a shady backyard with a pool. Lighting was all over the place really. I wanted to spend more time talking than looking at my camera, so I used auto quite a bit. I switched into manual for a few set shots of my son, but that was really it. I find Av and Tv will often pick odd combinations of ISO/shutter/aperture, and prefer manual to those in most cases.