I'm not sure I really like the fact that GTK has been ported to Haiku. Of course it's great when open source works that well and people put the work in to make something available for more people! For many more people than it has been before, Haiku has become a viable choice as their main desktop OS with Beta 4. Not too many apps are developed for Haiku natively and even the old BeOS apps, if you have one that you would still like to use, don't run on amd64. With GTK more useful apps became available.
But Haiku is a low-resource and especially low-latency OS with defined goals of keeping this spirit alive. It does still run on 25 year-old and older PCs. The few applications that exist for #Haiku you can trust to be much more careful with using available resources than the average GTK application. I don't believe that Haiku will become as wild as Linux. The OS still is much smaller and will stay that way. But if more and more #GTK applications will become the standard solution for tasks, there will be less gaps to fill and less gaps will be filled by new, small, low-latency GUI applications.
I'm not saying I don't like this development. I'm sure I'm going to make use of it. But I'm not sure I like it, too.
Edit: There is a blog post with a very interesting discussion in the comments about this: Haiku isn’t a BeOS successor anymore by Thom Holwerda