Tank should always be your main because he/she is the one that has to run and grab aggro if someone pulls off you, got to have that control to pull it back and stop your dps. If your tank sucks or is under geared you will wipe and fail. Do some googleing for warrior tank gear or pali tank gear and you'll find some great sites and tips.

Also many bosses require you to face the boss another direction or move the boss around to prevent from wiping party. You can't do this without making the tank your main. If you do not want to play a tank, just grab one from the pug system and run instants as DPS team.

I'm running (1) Pali and (3) Shamans leveling right now and it's all about the tank.