[dagw]

You don't hire a plumber by asking him about the make of his hammer.

You might if all the nails you have could only be used with a certain make of hammer and if learning how to use a different hammer efficiently could take several month.

[ricardobeat]

Let's not get carried away, but that's the result of your own choice to accept vendor lock-in. Like having to hire only authorised service providers. Most companies I've known are smart enough to avoid that and just hire good generalist developers, and the interviews will be shaped accordingly.