Depends on your point of view. From a GM perspective, you have to get these darned newbs some proper combat experience without sending them into a meat grinder. Plus it's a lot easier to say "Whoopsie, the bars just fell back in place and you're locked in now! Better find another exit!" than to say "Uh... you can't leave the forest to lick your wounds. You... just... can't. Because... um... OH NO MORE GOBLINSES!"
It's not entirely creative, but it works
For CRPGs... probably too much Waterdeep and not enough creativity
In Majesty though the sewers are just invincible random monster spawners that start appearing throughout your city once it reaches a certain size. They can be very problematic but also, by shear coincidence, provide a way to get some relatively safe experience for newbie hero units.