It should ideally hold skis and poles as well as other equipment such as boots, skins, ski clothing, tent, glacier equipment, etc., but should still be reasonably easy to transport despite its weight. So it should definitely be a large model with wheels so that you can pull it reasonably comfortably not only through endless airport halls, bus stations and train compartments, but also through the souks of Marrakech, the city center of Buenos Aires or the cobblestone streets of French mountain villages.
Of course, there are inexpensive models without wheels or other bells and whistles. Their only advantage is that they are cheap. If you're lucky, they'll survive a return flight, and often only the outward flight. With luck, the arm carrying the ski bag will also survive.
Even after many years in the business, I'm still amazed at how much equipment you can easily transport in a ski bag with decent wheels. The amount of equipment (at least for me) is limited, especially when traveling by air, by the airlines' weight specifications (usually max. 23kg) and not by the capacity of the ski bag. Anyone who has ever traveled with skis also knows that at some airports your luggage is weighed, you get the airline label stuck to your bag and then you have to take it yourself from the normal check-in counter to the excess baggage check-in. It is said to have happened that a few extra kilos have found their way into the ski bag on the way from A to B.
