I think what they might be going for in putting DoL/DoH on /random is to simulate how a given area doesn't just have only level XX mobs in it.

For DoH, anyway, you pick a recipe, let's say level 20, and you grind that recipe 100 times. When you go out into the field as a DoW/M, the enemies have a level range. You're not just fighting level 20s, though they're could be a good amount of them around. Usually the levels fluctuate slightly +/- 1 or 2 levels. This randomizes SP a bit. I mean, granted, you could specifically *only* go after whatever level you're aiming for, but the added travel and search time isn't going to make it worthwhile.

I could be giving SE more credit than they deserve, though, I'm just trying to guess at a logical thought process. I'd be perfectly content with static SP gains on DoH/DoL. The less /random there is, the happier I am (usually).