I use preshave oil regularly as part of my prep (and I think my prep is pretty good).

Been using both the AOS oil (mostly olive oil) and the T&H oil. I prefer the T&H because it is not greasy, not viscous like the AOS oil, and washes clean unlike the AOS. I'm only using the AOS until complete since it costs so much.

Anyway, IMHO, the oil adds a nice glide to the surface of my skin and also helps to protect against nicks with some of the less cushioning soaps. Is it absolutely necessary? No, but it gives be a little extra protection that I like. Yes, there are times where the oil makes my skin too slippery to stretch but I mitigate that as much as I can and it really isn't too much of an issue.

Really, to each his own.