I won't speak for others but there are a few things that do weigh in favor of the cost-benefit of HA:
(1) It has a strong safety profile. It's used all over in cosmetic medicine with reportedly low complication rates AND happens to be a naturally occurring substance in your own body.
(2) Somewhat reversible. I've read varying degrees to the extent it can be reversed with hyaluronidase, which could be useful for those who need corrections or for those who want to remove it. Fillers that rely on bio-stimulatory effects (Ellanse, PMMA, Radiesse) require more invasive measures to remove.
(3) For some guys, NOT being permanent is attractive. Not all results are equal -- if you aren't happy, you aren't stuck with it forever, nor will you have to worry about any additional measures down the road for removal. Plus, not all experiences are the same -- maybe your partner doesn't like your oversized girth, at least you know in a few years it will return to as it was before. You may think that sounds crazy but I've seen reports of bigger men being turned down quickies (vaginal tearing) and anal (self-explanatory).
In the same breath, obviously top-offs periodically can prove cost-prohibitive for some folks, especially if your body is the type to breakdown fillers quicker. If budget & logistics play a significant role in your long-term plans, then alternative fillers or procedures may make more sense.
A lot of guys have shared that they like the idea of HA being their "first time" filler. This way they can decide whether or not they even like girth enhancement before pursuing any additional appointments. They can also see if HA makes them happy, continuing with it every few years, or if they rather opt for more permanent options instead.
I hope HA guys chime in