I have not paid close attention to it, but I believe that as your level goes up, the experience rewards drop down. The credits may act the same way, but who cares, the real cash comes from sneaky diplomacy. The fame/fear rewards seem to stay constant.
If my guess is correct, it works very well for my style of playing, because less experience from quests means better crew members, and holding quests from enemies of your allies allows you to turn them in later, when you have maxed out the reputation for the treaty you are willing to observe.
I wish that one of the races offered a 'turn in quests through fake identities' service.
You'd get a lot less experience - some of it comes from being well known.
You'd get somewhat fewer credits - there are expenses.
You'd get little if any fame or fear - no one knows you were the one who did it.
You'd get no reputation boost or hit - for the same reason as above.
Why would you still do it? Some cash, and resolving the underlying problems. Sometimes you want to help a race without compromising your relations with other races. The good Drox operative engineers the demise of his enemies while still maintaining non-aggression pacts, and using their services.
No good deed goes unpunished...