I was told to go with the lowest temp rated sleeping bag that I could afford because one can always cool off by laying on top or unzipping. Freezing your balls off in the middle of the night and not being able to do anything about it is the worst.
Affordability aside, is a -25 sleeping bag used 4 season a terrible idea? What's the worst that can happen, that I don't ever sleep inside of it unless in a snow storm?
It's much better to have several sleeping bags, than one do-it-all bag.
A -25 bag for 4 seasons isn't a good idea really. It will be much too hot for the majority of the year. You won't sleep well, and wake up covered in sweat. If you're mostly camping in freezing temps, approaching 0 degrees and below most of the time, then sure a -25 bag will work. Most people camp more frequently when it's warmer though.
A -25 bag will be significantly heavier and bulkier than a warmer rated bag. If you never hike and backpack, then it's not a concern. If you have to carry the -25 bag everywhere, it'll suck.
It's better to have a sleeping bag for winter camping, an intermediate bag for early spring/fall, and a lighter summer bag.
If you do only want ONE BAG TO RULE THEM ALL. I'd say a 20 degree sleeping bag is best. You can wear clothes and get a liner for colder temps, or open it up and vent it for warmer days.
For me, I have a 0 degree set up for winter, a 32 degree set up for spring/fall, and a 50 degree set up for summer. I can take each set up lower or higher with a liner, or wearing less clothes under. So really I have myself covered from sub zero temps on up.
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DO NOT BUY THAT SLEEPING BAG!
I've owned that bag for around 5 years and got 2 years of use out of it before the zipper failed me. I about froze to death. Had to use a space blanket to wrap around myself then wrap the sleeping bag around that.