Ok, I am also a bit sick of all pwerlvling and leeching in high lvl parties. I would allow level for parties +/-10.

High lvls still can help low lvls, but : here is my point :

low lvl need to do something as well - engage mob, high lvl can (out of pt) can help him dps.

But there should not be lvl 20 and 50 in same pt by my guess, this ruin all fun and low , mid content.