Like... everything in WoW? Of course it's easier to have a tank and a heal.
However, I think that the different scenarii available are not designed for the same comp. Blood in the snow is pretty unforgivable if you have neither a tank or a heal while there is no reason to have a heal (or a tank) in the scenario with pirate ships.
We try to never run a scenario with a tank-heal-dps comp since any tank can manage his own survival in these scenarii and any heal should be able to keep alive a melee dps aggroing everything. Having both a tank and a heal is just a dps loss.