There's no I in team and everyone plays a role...healer being the most important...
Given that even PUGs can easily three-man any Tier 1 hardmode FP with a healing companion . . . not so much. If anything, the companions are better than at least 50% of random PUG healers, and you either won't notice the difference, or you'll have an easier time. And I say this as someone who has two healing mains and leveled both of them on nothing but PUG FPs. I'll be the first to admit there were probably times when my companion would have done a better job than I did. Maybe not many; but some.
It's possible to loltank most FPs with a companion and a very strong heal specced player, but the team will take a lot more abuse, and you have no real control over aggro at all. The DPS companions are about as mediocre at DPS as the tank companions are at tanking, and probably not as good as even a lousy player in most cases (and that's saying a lot, given the lousiness of some PUG DPS).
On the whole, if you're going to go into any random FP short one man, it's easier to fill the healer role with a companion than any other spot, and it will slow you down less than any other unfilled role. In operations, that might not be so true.