They may be unique abilities but I think the person you replied to was pointing out that whm doesn't really bring anything useful (<-- important keyword) that the other 2 healers can't. They can certainly bring big powerful heals but in content that doesn't need it it's kind of a waste. The other 2 healers can at least bring something that increases the overall rdps of the group which is what people generally prefer.
Edit: If they do something that increases the synergy between whm and the other healers I could see it working out though. Example a Deploy POM.
