It basically comes down to the individual playing. We have a vanguard and a guardian tank in my guild. I have a shadow but she is dps. Both our tanks know what they are doing and we have an awesome smuggler who is a healer.
We rarely have problems running heroics and flashpoints but we each know our responsibility and we back one another up when we can. We each have learned to play our toons and learned how to back each other up.
A tank, dps, healer is only as good as the player and them knowing their character.
For the most part I agree completely. At 50 each tank can be as good as another, if the players have the same level of understanding of their classes.
Pre-50 is where a lot of the problems lie. Guardian/jugg is much better at single targets, shadow/jedi equiv and power tech/jedi equiv are geared more towards aoe, with shadows/equiv having more ranged pulls and power techs/equiv having more raw ranged output damage.
As a healer, 9 times out of 10 the easiest runs I have are ones with jugg/guard or PT/equiv.