It's not common, but I wouldn't say it is "rare" now.
See, in 1.5 a bug was fixed. There is a 1% chance the event triggers, and then the game looks at 50 random tiles on your farm... and before 1.5 if even one of those tiles was blocked the game would not complete the event, but now when it checks the tiles, if even one of those tiles is UNblocked the game will complete the event. This makes a big difference. Now just about everyone will get one in a save if they play it long enough.
The chances for it are the same as the Strange Capsule event, and the method of checking as I described above applies to that too... and when rolling out 1.5, in 1.5.3 CA had to institute a "once per save" lock on the Strange Capsule... and I'm not sure, but I think he may have done so for the Stone Owl too.