Replace or repair derailleur? Typical situation.  Derailleur goes into a wheel (for whatever reason) and rear derailleur cage is bent.  If you were a customer would you rather have the derailleur replaced and know it is fixed, OR have a mechanic attempt to repair it by bending it back or replacing the cage and accept the problem may not be fixed the first time?

Yes, the replacement is more expensive, but is a sure fix.  Replacing parts or manual adjustment may be cheaper, but introduces doubt into the repair and in all likelihood may work in the shop only to fail in use.  Then a customer would return, upset the repair failed.  

My stance is replace it and remove all doubt.  Been asked as a mechanic to "make do" one time too often and want some outside input.  

Repair or replace?
Replacing it is the more economical option in almost all cases, especially on lower end stuff (esp. once you include labor) -- you can't source the parts for many lower end derailleurs.

On higher end parts it may be possible to send it out to a company which rebuilds derailleurs or rebuild it yourself with parts, but again, the labor and parts will likely outweigh the cost of just getting a new one (and the new one is less likely to fail; the rebuilder may make screw ups). 

So, I'd almost always replace, unless I had a good reason not to (e.g. someone who insists on vintage parts and wants that vintage part repaired). Or if its a tiny deviation (which wouldn't occur if the derailleur went into the wheel; that's generally toast).