[JohnFen]

I always ask this regardless of whether or not they're senior level. I think the answer can give a real insight into what sorts of things gets the person excited. If they don't have any programming side projects, that's not a mark against them, but I will follow up with asking them what their hobbies are.

[andyish]

I think this is the way forward, their answer wouldn't turn an offer into a rejection but it would strengthen their offer.

I must admit I also ask this question but have never been told not to and I find that every so often you come across a very senior candidate who nonchalantly says 'yeah, I built a plugin for abc that helps me xyz'. Then you look it up and they've got 500k installs.