As I wrote, in this case I didn't see any other air crew the whole week. So I think perhaps CI is the only airline currently using this hotel. And they don't even fly to VIE every day. I think I overheard the crew saying they would be there for 3 days.

But at other hotels where I have witnessed similar behavior it all about alcohol. I guess the crew lounges don't stock a lot of beer and certain crews like to drink a lot. So they sometimes do what they can to access the Club lounge. And of course the senior pilots get that perk anyways.