Can several independent facet systems be used at the same time?
I think that is the intention.
Why are facet errors thrown as exceptions?
I don't know.
Do the leaks affect code that doesn't use this?
No. It's the parsing of facets that leaves stack droppings and it is the facet group object that is leaked.
Are somewhat more extensive docs in the works?
http://pike.ida.liu.se/docs/docs/MikaelAmborn_Facet_Thesis.pdf
Where's the test suite?
Not yet.