In addition to that lxskllr mentioned, it is harder to trim in general with a blade, because it is shorter, can't get under objects, can't sacrificially (accepting breakage of line) trim right up against objects without damaging that object (fences, walls, etc) or the blade or trimmer itself, throws the trimmer head sideways when it hits something hard, and can be dangerous, plus you have to resharpen the blade, balance the blade, and clean the muck off so it doesn't excessively rust or keep it oiled.
Most of the time I am trimming something, it is to get as close as possible to an object that I don't want a blade to hit, which is why I am using a trimmer in the first place instead of just running over whatever it was with a mower.
Using a blade for thing it is really needed for, or hitting it against objects, either way is likely to put enough strain on the trimmer to wear it out prematurely so in the long run it is probably cheaper to buy more line instead. If you had a large lot with enough trimming to run through line fast, then a cordless wasn't ideal for that in the first place since they have shorter runtime and need more batteries to do the same work as a gas trimmer.
Remember, you go through line because the line sheared off from hitting something harder than it is... something you don't want to hit with a blade at all, more often than not. If you have to keep a blade further away to not hit something, you can do the same with the trimmer line too.