Rare? I've seen it a few times a year in my games.
I cheat, mostly.
I used to keep every scientist working by assigning them jobs, worthwhile or not, and one lab each. Couple games back it hit me that I could keep them working with zero labs each, so that's what I do now.
As a fairly new attention-conservation technique, I let new scientists sit idle until the next time a research project finishes up. So I wouldn't have many opportunities to see this message. It'd be easy for me to miss it.
Which all leads to a simple question: Do scientists skillup faster when they're working than when they're idle?
My tedious procedure of keeping every scientist working 24/7 would be entirely pointless if not, and I'd love to be done doing that.
EDIT: It occurs to me that I'm making a further assumption without any evidence: That a working scientist's chance of skilling up is unrelated to the number of labs he's operating. That's pure speculation on my part and I've got no idea if this is true or not. All I really know for sure is that scientists working projects with zero labs can get skillups with the same "experience" message that those with more than zero labs get.
So, the question I really mean to ask is: Is there any difference among the following:
- The rate at which idle (non-assigned) scientists gain skill.
- The rate at which working scientists with few labs gain skill.
- The rate at which working scientists with many labs gain skill.