I'm first of all excited by the fact that this made first page on HN. I didn't realise there was that much interest in Prolog news (as in more than two or three people).
Also, having more open-source Prolog engines is a very good thing, both for the language and for the project itself. For the project, it's good because it makes it easier for a community to grow around it. Swi has benefited enormously from its popularity (Jan Wielemaker has commented on this occasionally). For the language, it's good because the more healthy Prolog communities we have, the more we can hope to get new people interested in the language.
Comparing to other Prologs (and Prologs-ish'es) the problem with having closed-source and only for-pay Prolog engines is mainly one of portability, which in the Prolog world is a bit shit (and that's a vast understatement).
Frex, I wrote my degree dissertation using Win-LPA Prolog in 2010-11. LPA is commercial and I don't have a licence anymore (I had a student one) so now I can't run my dissertation. I tried (and tried (and tried)) to make it work on Swi but failed (and failed (and failed))[1].
The problem is there are subtle differences between implementations that bite you in ways you never thought possible, and it's just aaaargh hard to get the same program running in the same way on two different Prologs.
Apparently the exception to this portability hell is Yap and Swi, who have worked together to maintain some compatibility with each other. I don't have experience of this myself (after getting burned I've stuck to Swi) but if Amzi open-sourcing their stuff also means they'll be more open to this sort of working with others, then that's really, really amazing news.
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[1] It's here: https://github.com/stassa/Gleemin
But it will hurt you.
Also, graduate dissertation, 5 years ago: embarrassing.