I recently bought a pair of BPL Stix. I have yet to use them(!) and am planning a short winter trip - my first - in which I'll either snowshoe or ski.

Has anyone used these for skiing? Do they hold up? Are they stiff enough?

I've added some Leki tips and taped/shrink-wrapped the bottom 10inches to protect from rocks/ice.

Has anyone had any broken poles from similar activities?

If they're the right height for hiking/snow-shoeing then they will be far too short for cross-country skiing in my opinion. Cross-country ski poles should be somewhere near shoulder height to prevent you having to bend over all the time which will become very tiring.

I use adjustable Black Diamond Traverse poles that can be adjusted out to 155cm (I'm 184cm tall).

Unless you are a very good skier (and never fall) then I wouldn't use them. I know some of the Cross Country ski racers use carbon fiber poles, but my guess is that the poles are thicker and the skiers are much better than us (and traveling on groomed terrain). I've broken trekking poles while snowshoeing. It is very easy to sink a pole into deep snow and then put pressure right in the middle (where it is weak).

Joe is also right about the length. Black Diamond makes a lot of very good adjustable poles that work well for Nordic skiing. One of the really nice things about the Black Diamond poles is that they stay locked (which reduces the chance of breakage). I would get the ones designed for winter if you go with Black Diamond. I saw a friend break a pair of Black Diamond hiking poles while crossing a creek, so I could easily see how that could happen while playing in the snow.

+1 on the BD Traverse poles! I used these on a XC skit trip a few weeks ago and really liked them.