My first Job I ranked to 75 in XI was DRK...that was during RoZ/CoP time, so you can figure how much I got "excluded" from stuff. That's the time where you need to take the initiative and step up and do shit on your own. Form your own groups, start a group, make a LS, whatever...it all comes down to people complaining but doing NOTHING to change it. I started almost all of the groups I lvld my DRK with and big surprise, over time people got to know me and actually ASKED me to join their PTs, cause they knew I've been a good DRK compared to so many shitty ones who actually worked on his job to get good gear and have Scythe/Sword both capped etc...
And why? Because people are lazy and rather have everyone else organize shit and just expect to be taken to just everything. Sorry, if you're not stepping up don't feel butthurt if other people who actually do it, won't let you join.
Plus, if you wanna come on Dungeon Runs as a Meele, which is absolutely possible and works, why not actually DO something and help the group by being prepared...rank your classes so you can avoid dmg and don't need to run from every single AoE attack. Get Stun WSs into your bars, get Featherfoot, Foresight, Second Wind, Decoy, Diversion and whatnot...ALL of them will help you not taking dmg while staying on Ogre and Batraal so you won't be a "burden" for the healers and instead a valuable addition to the group.
People could find solutions to counter this "narrow minded" group setups, but most are not even willing to do it, cause they are lazy f**ks...
If you want to go into the dungeon and KNOW just your mere presence will make it harder for the rest of the group, why not get the stick out of your ass and actually improve your character and his skills to be less of a burden?
People just screaming and complaining "I want to go in the dungeon as well" and then being excluded from it just cause their Meeles are just as selfish, as everyone else who excludes them for no reason...
PS: Your Canopus Bill example is invalid, cause MRd is a viable tank on Ogre and Batraal...if you got MRDs who don't wanna tank, well then don't be butthurt as well...work with what you're given!