I'm pretty much against the devs really deviating from the trinity system because, as others have already stated, the balance issues would far outweigh the theoretical positives.
I played another game known as Dragon Nest, an action MMO, that originally started with something that resembled the trinity system somewhat (although there were never role restrictions in parties at all), although once they began releasing new classes that deviated away from the system (as in having hybrid roles), it resulted in massive balancing issues for YEARS that slowly drained the life out of that game. You had healing classes able to out-DPS half the classes in the game AND out-heal the other healers for various reasons (such as faster-casting/longer range heals). Some of the top DPS classes in the game were capable of tanking better than actual tanks, along with having party buffs and debuffs that were so good that the developers had to implement a cap on how high stats could go/how much enemies could get debuffed (which only made the situation worse, as people min-maxed party compositions around them). And the devs would implement mechanics to discourage abuse of the buffs/debuffs such as buff wipes and numerous boss invulnerability mechanics, but all that really resulted in was a 'burst to skip mechanics or die' mentality among the community.
On top of that, the power creep and the DPS checks soon became so insane and the gap between what classes could do in parties became so vast that clearing a raid eventually became a question of what classes you were taking, rather than anything to do with player skill. Even regular solo content became near unplayable for the 'pure healer' and 'pure tank' classes because the hybrid classes and the out of control power creep pushed damage standards unreasonably high. It was so bad to the point where the developers in the past half year decided to basically reboot every class in the game by almost universally nerfing/outright removing some party buffs, debuffs, and heals, redsigned most content, scaled back on the ridiculous mechanics, and actually pulled back into a trinity system. And for the first time in years, people now consider that game's PvE to be -playable- for the average non-raider. Although admittedly, those changes were an extremely hard pill to swallow for the majority of that game's raiders, although most now admit that the changes were necessary in the end - especially when the vast majority of raiders in that game had been changing their class every half year once things began spiraling out of control from what I had seen myself.
(Given the action nature of Dragon Nest, everything in that game is basically varying levels of DPS as well, much like how people would describe healers and tanks in this game. But completely disregarding the trinity system would be foolish in actual practice, while any advantages are only theoretical at best.)