[asdff]

It's not like derailleurs are plagued with issues or costly maintenance as it is. It takes a lot of riding to knock them far enough out of spec where its a problem, and a tuneup at a bike shop is like $25. Pay that once a year and you will have like zero problems with your bike unless you crash it badly. If its a matter of protecting the derailleur from damage like on mountain bikes, you can install a guard for a few bucks.

[Valgrim]

I see them useful for winter biking in the city, where corrosion and dirt is the biggest problem.

[asdff]

Most people who I've seen winter biking are riding some beater $100 used mountain bike into the ground during winter over a nice bike. Remove the derailleur, and there are still a lot of components all over the bike that will get fouled up with corrosion, dirt, oily sludgy snow, etc, so its better to ride something disposable during that season.

[Valgrim]

I agree. My personal experience is that the chain and derailleur are by far the most fragile parts of the system (disk brakes are probably quite fragile too, but regular caliper/cantilever/v-brakes are not much affected by corrosion.) but on cheap bikes, chains are easy to replace and derailleurs have a lot of tolerance. So just cleaning them regularly and oiling them is generally good enough.

Some specialized winter bikes use a belt instead of a chain, but I find those too expensive, hard to service and maintain.

[ubu7737]

You can replace a derailleur hanger on a modern bicycle for about $10 or less. This is the part that bends on a right-side crash.