If I need professional tools (or arts, music or whatever) I have to pay for it. Sure, there's a lot free stuff around, but especially if you design a product like Stencyl, you need reliable partners you want to cooperate with / support their tools.
It's not funny if you implement a free toolset into your toolbox and after 12 months the open source developers start studying or get bored or whatever and stop the development.
I am fine with paying for Spine if I do need this kind of animations (which might happen one day). It's another 60$ on top of my toolbox, that's okay. Spriter will end probably around $35 to $45 (except with a free version), but Spine is afaik the most mature and most supported tool-set.