You'll quickly learn that you won't really be tanking. Once you're all experienced you'll find that you could easily be doing more damage without taking the hits anyway. While aggro works off toughness, you'll never really prevent anything dangerous from reaching your party that already targets the entire group. The only exception is Lupicus, which doesn't require any kind of defensive gear anyhow since bunkers trade all there damage to get hit one more time.
Dodging honestly negates just about all the damage you can ever take, if you really want to soak up damage and still fit in an optimal setting stack atleast 1,500 toughness and use a gs + axe/shield for the ghetto dodge and shield block. That should keep enough attention from bosses and your damage won't really suffer.
If you're honestly dead-set on tanking, and just doing it for fun, you should go for mace instead of sword so you can eat up defiance stacks quicker with the extra CC. And keep in mind, for any type of supportive play, for a boss' ability to really be interrupted, you have to wait for it to actually go off before you interrupt. Rupting the cast animation just puts it on a 3s CD.
I know I can't become unkillable tanks like in other MMOs, but if I can absorb a lot of damage to support my group I have done a good job. Obviously it is not like in other games where all the damage will be focused on a single target, but there is not only the role of dps.
Sad to say it but full DPS builds with experienced dodging are better defense than defensive stats. The faster you end an encounter, the less damage the mobs throw out, the less your team has to dodge, and the less everyone will mess up. Soaking up damage doesn't do much for your team, as there's enough ways to soak up damage without any defensive stats at all.
Pure damage for every class has done it best in pve since beta.
I won't debate with you; guardians running knights gear don't do as much damage as guardians running berserker gear, provided the player is experienced and doesn't die. My point was that inexperienced players running berserker gear die easier and more frequently, resulting in a much lower contribution to DPS on runs.
An inexperienced zerker guard running the right build/set up will do the same, if not more damage in their short life than as a knight guard that can still be downed by most of the content.
You don't really learn how to play the most effective way in knights armor, guards in zerker gear have more than enough survivability if they use even just a scepter, and they don't gimp their team's DPS by nearly as much.
You are however talking about player-made restrictions. And we can't really blame ArenaNet for those.
Looking strictly from a game point of view every single class can do every single (game created) thing in the game.
Most restrictions in games are player made across games, they took away all but DPS in pve, so since everyone can cause damage then they're all viable. Anet made viability a cheap way of saying they stuck with their own manifesto.
Which is very much true.
I have yet to find a single thing in this game that requires a specific class setup.
I think they were lazy in implementing this, as 2 decent players can usually carry everyone else through a dungeon. And all the classes generally play the same, ranged or melee full DPS. That way they can easily just say "they're all effective".
CoF and some fractal groups will require specific classes/group comps. And the typical CoF warrior/mesmer set-up can be used everywhere for the best run results, if dungeons actually gave above-meh rewards, then you'd see that kind of BS much much more often.