You make good points, but I think they might be mostly applicable to someone whom you expect to be productive day 1, as in a contractor or on a project with an extremely tight deadline.

I have been in the microsoft ecosystem for many years, and then I did an internship at a Java shop. Granted, a lot of the things there were new to me, but most of them were parallels from the asp.net world. Even if you are looking for a senior dev, maybe start them out as intermediate for 3-6 months, and then re-evaluate them to see if they've caught up and deserve to be made senior at your shop?