Thanks, everyone, it's useful to know that some MSCs do this.
I do feel it's a little different than having me buy the ticket and reimbursing me for it. In that case, they're asking me to do it either because it's convenient, or they want the ticket purchase process assessed, or both.
In asking for a deposit, though, they're telling me they don't trust me in spite of my track record with them. I know it's not directed at me personally, but I'm just not interested in doing business that way.
No harm, no foul, then.