It honestly depends on several things such as the type of forum you're using in my opinion, and if it's the same genre as the other. Webmaster/Promotion sites will get members easily if promoted on other webmaster/promotion sites. However, if you're promoting something other than that you may get a few active members here and there but that's about it. Most people are on there to promote for themselves, however there's those members you see around that are actively engaging in the community. I think if you're one of those members who are active around a lot of the sites more people will be willing to be active on your site in return. It also depends on that websites marketing strategy. Over time I will be building the SEO for a few keywords that will bring members LOOKING for websites in, versus just promoting. If other sites do the same, then it will be worth it to advertise on other forums.
All in all though, any advertisement on another forum is still at minimum 1 extra backlinks if they're Do-Follow, which means if that site gains popularity that backlink increases your overall SEO so I do believe it is beneficial to promote on forums as long as they allow it