I'm confident in the fact that any starter, no matter how small, is far better than no starter at all.
Fortunately, when my standard batch size is just 1.7 gallons, that's like a big starter by itself, and thus I usually skip the starter. I only make starter for kolshes, alts and lagers.
Also, I try to use dry yeast whenever possible. There are dozens of great dry yeasts these days, such that you often times don't need to use liquid yeast anyway, and of course you don't need to worry about making starters with dry. Huge advantages to dry yeast.
So, yeah. Even if you're "underpitching" according to MrMalty or whatever other expert out there..... if you're making a starter at all, you're miles ahead of the people who just throw in a vial of yeast unstarted and hope for the best. Many of the worst beers I've ever made were done that latter way. I don't do that anymore when I make a big 5 gallon batch (which I still do a couple times per year).
As for oxygen... when I make starters, they're shaken. I do not own an aeration stone, stir plate, or oxygen tank. Shake it up a couple times. Works great.