I apologize! I should not have just complained without making a suggestion as to how this situation could be improved... So here it goes:
I favor allowing all players to have access. But not all players should be rewarded for participation in DSEs. I'm pretty sure all of these "Defend the Sector Block" missions have intended level ranges. Keep the NPC you encounter within the level range for that sector.
But limit the experience and drops one gets for participating. Specifically, If a player is more than one Rank above the "intended level" then they should get "No Experience" and "No Drops". This type of system won't allow players to 'power level' their friends.
They should still keep the 30 minute timer for completing a series of three encounters. But it should eliminate the scenario where bored Admirals hop into a DSE and mess that instance up for everyone else.
As an Admiral (whos not a power gamer as the previous poser suggested out of anger), i did one of these once, ***** it. The rewards are scaled to the level intended tho, the only reason there is for admirals to go in there is to either farm drops to sell or grief lowbies. Leveling doesnt take long enough in this game to warrant my 'helping'. If you cant manage to level in this, seriously, why are you even playing MMO's?
(i know you didnt say that you couldnt, just pointing it out, dont mean 'you' as in literally 'you/yourself'.)
Obvious oversight by Cryptic on this, just have w/e DSE someone tries to get into send them to one appropriate for their level, problem solved.