Tanks and Supports should be allowed to deal (appropriate) Damage.

If, as some people suggested, Tanks are reduced to CC only without dealing any kind of significant Damage i believe it will reduce the ammount of people playing them.

I mean look what happens to the support role who can´t do much once their team is down. You currently have to build full utility in higher elo on most enchanter / Utility Supports. But what happens if your Active Items are used, the summoners are burned and the team mates are dead? Youre a running caster minion with a slightly better bounty on its head. Even if its an Ace from your side with everyone dead but you still alive. Theres nothing to do, except clearing wards / placing new ones.

This sucks in many cases and is, in my eyes, quite a big influence on why this role gets no real recognition.

Damage dictates everything in this game. Every good Bard/Nami/Soraka Ultimate is just a set-up for the bigger Damage-Dealing play.

Some scarce individuals might find enjoyment in this part. Being the enablers of the team.

But dont you feel helpless if you have a team full of boosted bonobos? What are you supposed to do if your vayne has a tendency to tumble right into the fed kha´zix? Heal? Disengage? Exhaust? Sure. Thats nice and all but what if all of this is not available and a bit of damage in your build could make the vayne win the 1vs2. Well too bad but your not dealing damage.

Same goes for Tanks. In case they deal no damage and are reduced to Utility / Defensive Items, what will you do if you miss your stuns/snares/knock-ups or the enemy team just dodges them? They will ignore you and just kill the real threats.

Of course there has to be a trade off.

More Damage = Less Tankyness.

Is 14% HP damage as Base too much? Maybe.

But if you nerf it quite heavily you get a unkillable scuttle crab.

Maybe useful for pro-play where a team can play around such things and make use of the Stuns the tanks provide and balance out the horrible laning phase against everyone who does damage.

But in Solo Queue play their playrate would plummet quite a tad.

So.

Tl;dr:

Having significant Base Damages that let you deal with minions / pesky jungle creeps or just to not be ignored in team fights is important for both Supports and Tanks. There might be a slightly to high base damage on a certain Tank *cough but damage itself is necessary.