I don't recommend talking with the other tanks and asking for a spot without talking to the officers first.

Just because your gear is equal, doesn't man your skill actually is.

I used to have in my guild someone who was a shadow priest main, but really wanted to switch to his Death Knight. He went to one of my tanks and asked him if he could tank runs, and got all this permission from him, and suddenly one of my tanks was logging on to runs on his alt, and my shadow priest was on his DK and both were wanting an invite. The tank's alt wasn't so bad, but the shadow priest's DK tanking was horrid and he couldn't hold aggro over a mage evocating.

Generally, it's nice to have backups. If a tank suddenly gets a bad connection, or has an emergency or has to work that night, you have an extra one or two if you have five tanks. But the problem with having five tanks, is each person tanks differently, and each healer you might have has to learn how to heal them. And each healer has to learn how to heal each tank in each fight.

(Number of Healers)x(Number of Tanks)x(Number of Fights)=Time Where DPS WIll Be Able to Do Their Jobs Without Dead Tanks.

Honestly? The guild just likes having the guy as a backup. He should Pug, or go find a new guild. Or with 35 people... 35-25=10! Go do a ten man!!