Highly depends on how much you're paying attention to those numbers.
In a vacuum, you're right. But if you know you were doing 14k yesterday on a dummy, were pulling 13.5k in a dungeon, got a new piece of gear and then am doing 15k in this raid, you have a gauge for your normal performance and where you fall personally and can see if you're performing better or worse than you normally do.
Also, with a personal tool like that people could still post logs and you could see how you stack up against other people.
I agree it wouldn't be as useful as a full group DPS meter, but it would be far from useless.