I paddled a Bell Magic for many years. My doubled-blade paddle served best in about 95% of my travel. But in twisty creeks and approaching rocky shallows I switched over to my single blade, just to prevent damage to the DB. And obviously the single blade served as a spare/backup in case the more 'delicate' DB suffered damage. Admitedly, the single blade had a shorter shaft than ideal, just to save weight and space.

For me it depends on the choice of boat, whether or not I intend to troll, the remoteness of the route, and whether or not I'm trying to go very light on my carries.
1) Some solo boats, i.e., pack boats, arse-on-the-floor, aren't configured for efficient single-blading
2) Trolling without mounted rod-holders that serve to extend the fishing line several feet from the gunnels (and out of paddle's way)  makes double-blading inefficient and annoying
3)  Degree of remoteness is a factor that requires one to consider bringing a back-up paddle.
4)  A rarely-used 2lb.+ additional paddle might be unnecessary cargo on a short, quick, heavily-travelled route
5)  My trips are always at least a week long and fishing is important to me, so I almost always have a single-blade with me and a double-blade.  When solo-ing tandems I forego the double-blade altogether in favour of a 2-piece SUP (stand-up) paddle.
6)   Paddling canoes, even "dedicated" solo canoes doesn't feel quite right to me without incorporating some single-blade paddling, i.e. hit-n-switch or traditional Canadian-style into travel time.  I like switching up body positions and paddling styles and find I can paddle longer and with less fatigue by changing things up.