Hoi,You are factually wrong when you say that MediaWiki does not scale.. if anything it is one of its strenghts. After all, it runs one of the biggest websites in the world.

Also I am sure your commercial alternatives do not have the same kind of multi lingual support. As it is MediaWiki is second to none and with the LocalisationUpdate extension, the localisation can be improved on on a daily basis.

As to usability, the WMF has its Usability project going, it brought us already the first improvements and they already make a difference.

All in all, when you need to talk commercial wikis up, get your facts straight.Thanks,GerardM