Which quests are you talking about? There are some quests that are intended to be repeatable (exploration quests, enemy signal contact quests) and some that have been repeatable just because of completion bugs that cause completion not to fully happen.
Also if someone shares a quest with you and you were not 100% eligible for it at that time, then you will get diminished rewards. You can pick it up again when you are eligible and get the complete reward for completing it that second time.
Just because a quest is repeatable doesn't mean you have to do it repeatedly.
Exactly. Now granted I've only made it to Lt Cmdr 1, but I only repeated a couple of exploration missions because I felt like grabbing some Badges, meanwhile I haven't even done half of the sector patrol missions I've gotten so far, and probably never will because I have higher level ones to do instead.