On double healer teams I always went for a healer.. usually after a thunder storm or some fire nova totems that put eveyrone's health down a bit. even with cross healing it was an instant kill on the healer. Those teams don't have enough dps to burst me down without their healers.

On 4 dps teams (or 2dps in the case of 3's, i guess) it's easy to take out the most threatening dps.. usually a boomkin or mage. A single healer just couldn't keep them up.