A lot of Wordpress stuff out there is premium and rather expensive, so to see quality Wordpress tools that are free is indeed an important feature. Most free Wordpress stuff is ugly and useless (except for Wordpress itself, of course).
In the WordPress world, it seems that "free" carries a lot more weight than "open source". While there are many who care, there's also a ton of folks making a living (or just running their own website) by connecting together plugins. I'd make "open source" a bullet point, not a title-level feature.
I agree. Given this particular forum I thought the free and open source aspect of it would be more interesting, but you're right: it's definitely not the drawcard of Tailor. That said, the benefits of it being free are obvious and being open source people can get involved and help shape it (like WordPress!).