"What's wrong with oil?"
Ultimately, there's nothing wrong with oil used to lubricate the slide rails on a semi-auto, except that I don't much care for it for a couple of reasons:
1. I wear glasses, and inevitably some of the oil is going to be chucked back onto the lenses of my glasses. I hate having anything on my glasses.
2. It tends to "go away" a lot faster than a light-bodied grease.
If I'm in the middle of a match, though, and the grease starts to crud up or wear thin, I'll often lube with oil as a temporary measure.
I generally prefer grease for enclosed places, especially the interior workings of revolvers. Grease stays where you put it, oil generally does not. It flows over time, and invariably it flows away from where you want it.