The problem with op gating is you shut down the number of players who are available to play. If a guild is on to the next tier of Op and three of their raiders (for example) have various real life issues to deal with, finding replacements is infinitely more difficult and requires the guild to go back and run the older instance and hope for the right drops to get the replacements up to speed.
On the other hand, if you aren't gating on gear you can carry the replacements for a little while until the current content drops catch them up.
Many don't think so, but it really is a good thing that so many people are getting geared up on ops and BH gear. It will benefit the top guilds whether they think so or not (lots of good players who were behind in gear and couldn't necessarily catch up to the point where the top guilds would give them a shot.)
yep you are slowing things down aren't you? kinda what i am getting at. might want to have a raid with people you can swap in and out to gear up to eliminate this. we did it in another mmo that had gear requirements. took a little longer sometimes but had our own replacements and allowed more people to raid. worked rather well for us. there are solutions to your gating gear requirement problem if people just think a little or had experience with it.