I use a half-cover every day commuting and also when traveling.
At first I was bothered by the time/ nuisance/ effort of having to don and doff the thing and it's bulk when rolled up half-heartedly. Now I can't imagine living without it, and I can deploy it or remove in well under 10 seconds.
Keeping the rain and UV off my spendy aftermarket seat was my primary motivation - my bike sits at least 9 hours/ day during the week at work.
The half-cover (as opposed to seat only) also keeps the UV off your plastics (paint fade), windshield (lensing), grips/hoses/ cables (dry rot). It's worth the extra bulk of a half-cover vs. a seat cover to me. Again, my bike sits out in the weather a LOT. I wouldn't bother if I didn't commute.