What is ski erg?
Ski Erg is an indoor trainer (from Concept2) There are a few other brands, like Ercolina. You can snoop around online in general, but also here on ST, about features, utility, opinions, and so on.
I think it is as valuable for skiers as a bike trainer is for cyclists (and as a rowing machine is for rowers). For a couple of reasons. First, skiing effectively requires a lot of upper body strength. (Ski cruising, maybe not.) Many cross-over skiers, especially coming from a running or cycling background, under-appreciate this, in my opinion. Triathletes with a decent regimen in the water probably don't have the same deficit to overcome.
Second, a ski ergometer is the only "measurable" device in the whole endeavor. There are so many un quantifiable variables in nordic skiing that you are left with heart rate and RPE as your only metrics. Distance and terrain, temperature, snow humidity, grooming conditions, wax prep. So many things you cannot control and cannot measure. But a ski erg is watts. No more no less, no better no worse. At first, I modeled my workouts on swimming. Later, I switched to TR.
Third, depending on where you live (Colorado, right?), your season might be 10 weeks or 20 weeks. A ski erg, like the roller skis we have been discussing, extends your season. Additionally, there's travel for many people. I happen to live in a town with probably 50k decently groomed trail, and 2.5k world class groomed plus snow making capacity. But if I were facing 30-60 minute drive to ski, heck a ski erg suddenly sounds pretty convenient.
Fourth, having a ski ergometer means you can "split" technique from fitness. What I mean is, you can go bonkers on the ski ergometer during the weekday mornings. Then hit the trails on the weekend and focus on skills without worrying about . Or whatever.
Finally, any ski ergometer is expensive. About $800. A lot of people are going to shy away from that. But that's just a nice set of wheels, or even just a single top end wheel. And top of the line skis or boots run the same price.
All that said, I think I am in the minority. Most of my ski buddies do not have ski ergometers. Or they are lying. Which is a decent possibility.