That's the glory of the leveling system. You do just as well against an enemy you're 30 levels higher than as you do against an enemy you're on level with. The only reason you seem so much more effective against those level 4 dregs at the divide is that you have a fully upgraded legendary gun instead of a common gun or whatever.
Levels only penalize you if you're underleveled. Over leveling doesn't help, so level 30 is useful pretty much exclusively for the hard raid. (:
Also possibly the weekly and nightfall strikes. I know they're level 28, but maybe a few enemies or the boss wandered above 28 just to check it out and see what it's like.
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IMO its better better to play strikes that are 1 or 2 levels above your character to maximize your chances for better gear. So long as your fireteam are good enough to pick up your slack, it's why I mostly play them with friends.
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Yes! Why would a 29+ need rare tier garbage?
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Its not the over leveling thats the problem If I can beat the hardest content Destiny has to offer at level 28 (Im 30 now) then there wont be any new challenge