Depending on how you define "100%" Damage Meters across the world disagree with you, the only classes that can claim to function at 100% all the time are the Main Tank, Rogues, Priests, and .. Resto Druids. An Off Tank may be at 100% while handling an add, but when it dies, even with a good weaponswap, he becomes a ~25% DPS, we may not be able to reach 100% as a druid, but in tanking gear we can do better than a prot warrior at DPS (This is assuming AQ level gear, where we actually get a half dozen feral upgrades) Warlocks have to life-tap, bandage, and sometimes stop DPS alltogether due to threat, hardly 100%. Hunters feign/drink, and are currently (it seems anyway) only capable of just breaking 600 DPS, mages must Evocate and hold back on their DPS (gg ignite), same with fury warriors. Very few classes are capable of functioning at 100%.
Plus your maths or adding 90% + 35% is flawed. You are NOT doing BOTH at the same time...
If you are 50% tanking and 50% healing you are functioning only at 62.5% (doing maths on the fly) compared to another member in your raid who is functioning at 100% all the time.
As for doing "Both at the same time" No one is capable of tanking and healing at the same time, not in endgame at least, Tanking and DPSing at the same time is extremely risky (though technically, bearform Does DPS and Tank at the same time, I'm doing about 250-300 DPS while tanking) Conquerors would allow a warrior to do both I suppose, but only to the same degree that we'd be doing, You don't tank with a 2-hander. Classes that can do DPS and healing? Paladins can melee to keep up JoL, Priests VE, Shamans drop Healing Stream Totem, and ... We'll get ILotP.
The "Both at the same time" concept is fools talk when including healing, except for those 4 cases, it cannot be done. Yes, Shamans, Paladins, and Priests can all cast heals - but doing so stops their DPS briefly. To quote you "However, one must realise there are TWO mechanisms already in place to prevent abuse. One, is the mana cost ... Two, is the restriction of talent points." There is a Third .. Time. Any healer that has been to Patchwerk will tell you how deadly the Global Cooldown is, any time you spend not doing one thing, you aren't doing it ... it's no different from off-tanking and then healing when your mob dies, it's just spreading it out (which is less efficient btw)