Well it seems to me that alot of people, me included, pitch the whole starter while it is active and I am wondering if there are, or why there aren't, those off-flavors associated with underpitching.
Pitching the whole starter or just the decanted yeast is a different discussion than over/under pitching. Underpitching is using too little yeast for the gravity/volume of your beer. Overpitching is using too much yeast.
Okay so then how is underpitching different than pitching your starter wort with the yeast? The yeast still goes from x amount of cells to y amount of cells in both cases, and in both cases the flavors from that growth are left in the beer.
Apples and oranges. You're talking about whether to pour used up beer along with the yeast into the wort, which is a whole different animal. It has nothing to do with pitching rates, and the flavors are completely different. It's almost like saying "whats the difference in pitching a vial of liquid yeast vs. a vial of liquid yeast plus 2 past-date bottles of Corona."
Note that it's different if your starter is at high krausen -> there are cases where you'd want to do this (e.g. you forgot to make a starter the night before). In this case yeast is still in suspension, and the starter wort isn't completely bad.
Okay. So it isn't the growth of the yeast but it is the rate of growth that affects the flavors it produces.
Correct. With yeast, growth = reproduction. The faster they grow, the more they're stressed. Stressed people put bad odors into the air, and stressed yeast put bad chemicals into the beer. Also, yeast that has a lot of sugar available eat a lot reproduce a lot, and poop a lot really fast. Just like when people gorge themselves, it can be nasty. And some of those nasty chemicals linger ...
If they grow really slowly then they poop out very little flavor chemicals (good or off-flavored). Different yeasts produce different flavors.
In some beers you WANT off flavors and underpitch intentionally. The clove/banana flavors in hefeweizen and belgian beers are off flavors. If you overpitch those they'll be blah.
By the same logic you don't want that your American IPA to taste like banana - it'll clash with the hops. And the american ale yeast's off flavors also include paint thinner, bad medicine, and band-aids. Not good eats.
Note that fermentation temperature has a HUGE impact on this as well, but that's a different post.
When you're making a starter, you typically have a 1.040 wort, so you're overpitching if you were to think of the starter as a beer. Starter beer tastes bad because it is intentionally oxidized, not because it is esthery or has diacetyl. Unless you have a very high OG beer and want to mazimize fermentation start, you're better off cooling / decanting the oxidized beer.
This ^
Isn't the hops (or the hop oils for the lack of a better term) in the beer the source of oxidation off flavors?
And for starters, its typically made with just DME. So can starter wort even generate off flavors from oxidation?
Oxidation comes from oxygen, so you can get different off flavors from anything. Bad (oxidized) hops, old stale DME/LME, etc. will lead to off flavors (skunky, stale, etc.). However, putting oxygen directly into beer after it's done fermenting is the main culprit behind oxidized beer flavor like butter and wet cardboard.
