ACF
To be honest the lack of their own aircraft was something that I questioned too before I accepted the offer of a place on the internship program at ACF.
But the more I thought of it the more it made sense to cross-hire aircraft form another company. No coslty overheads such as hangers/parking, no maintainence costs, no need for any extra specialised employees, etc.
I'm still a novice I'll be the first to accept, but not really sure there are many advantages of owning your own aircraft other than you can have priority over them. However if a contract is signed and they are booked out well in advance by the hiring operator then there are no real problems I can see.