Most non-competes aren't enforced because they can't be (due to the reason you mentioned). BUT it still adds a level of risk to people looking to jump ship.
And many firms just don't want to deal with the potential hassle/exposure of hiring someone who has a non-compete that could potentially come into play.
In a former role, one or two of the larger firms in the space were known for having non-competes they were known for trying to enforce that basically said that if someone left--even if they were laid off--they couldn't do the same type of job for a year. Someone I know who left one of those firms just took a year off before going out on their own. The firm I was at wouldn't even think of touching someone to whom those non-competes applied.
