Drawback of currupted skill gems

I often go on poe.trade and see 20 quality corrupted skill gems that are considerably cheaper than their non-corrupted equivalents. Yet I see no difference between them, since as I read corrupted gems can still be leveled.
Am I missing something?

Hey there,

If you've a level 20 skill gem with 20% quality and you use a Vaal Orb on it, it can potentially become level 21 or have the quality increased to 23%. Both of these are improvements, naturally. Corrupted items cannot be modified by currencies, including Vaal Orbs.

Skill gems with either of those corruptions will be worth a lot, and skill gems which aren't corrupted at all can potentially get one of them. Corrupted skill gems aren't bad or damaged in any way, you just can't get lucky with them so whoever corrupted them will try to sell them cheaply to get rid of them. They'll function just like any other level 20 20% skill gem would function.

Not only that, but there is a niche case or two where the Corrupted modifier actually helps you: Skin of the Lords can only have Corrupted skill gems in it!

(But this has almost no impact on market price of Corrupted skill gems, as Lords is very scarce and not commonly desired; the prevailing market force is exactly as Sarno described, which is that non-Corrupted 20/20 gems have the power of potential for 21/20 or 20/23 behind them. The drawback of Corrupted gems that aren't already 21/x or x/23 is that they are missing that potential; the difference in market price is equal to the market's value of that potential.)