In reply to Dan Goodwin:
So... it really boils down to the airline rules/defintions...
I haven't flow in a few years with skis... so take with pinch of salt... exact details can be found from the airlines webpages about it. But I recall there has been three (or perhaps four) different practices.
1. Skibag is in addition of you checked luggage, some limitations might apply (e.g. sub 9kg, can only include a pair of skis, poles and boots). But free of charge... sometimes prior notice of these is required. Lufthansa and I think Finnair used to have these kinds of rules.
2. Skibag is considered sports equipment... so in addition to your normal checked luggage... and you need to pay for it extra... again airline specific requirements, about size and weight etc. But generally speaking they are max 23kg... so if you have a big skibag, you can actually cram your clothes there are well and only need that one bag...
3. Skibag is considered normal checked luggage... if you only have that to be checked... again airline specific limitations (weight, size... and also charge). If you still need to check yor normal luggage, see option 2.
The real difference is that for option 1. getting the skis onboard is rather easy and not affecting other stuff or an additional cost (put the tickets might be more spendy).
Options 2 and 3, are more iffy (read the fineprint and FAQ from your airline carefully). Extra costs might be there, like some companies (cheap airfare) stated that in order to get sporting equipment bought, you also need to buy the checked luggage... even if you had nothing more than the skibag.