What dire said was a good point.
If people will make a job as technical as thief work for BCNMs, it won't be pickups.
My static went pld/war, sam/war(dire), war/sam, smn/whm, blm/whm, brd/sam for the snoll fight for example. Maybe some won't, but I thought the setup was slightly strange.
We felt pld was a better tank for the snoll fight, so my ninja didn't get a chance to feel useful for a burst damage fight. Never tried thief altogether although I might've tried thf/drk if the brd/sam idea didn't work out. Instead we used me on brd to SV minuet the melee, minne/ice carol the pld, and ice carol to the mages with int etude for the blm.
I also charged to 300tp for spirits within for after we engaged too to chip in 500+100 damage. I'd imagine I'm one of extermely few at most to go through that as a brd/sam despite the fact I had 3jobs to 60+.
My point is, pickup parties won't think of ways to use somewhat unusual tactics efficently. You'll get stuck with idiots and inflexible people unless you're lucky. And using a thf is considered unusual often times when it comes to BCNMs. You'll want a static with 6 compement people, especially if most or all of them have 2-3 or more jobs to level 60, allowing for flexibility. Being flexibile is pretty ideal for CoP afterall. If you're going to be forced to have to get by with pickups and semi pickups, you're well off just leveling a muscle job. Using a thief efficently as pere stated requires more thought process, and more brains than the usual "Accepted" tactics. This isn't a bad thing, but you can't really expect it much from total strangers.
I went thf for some of the CoP missions, as I did with drk, ninja, and brd. I got to play my higher up jobs equally throughout CoP, using what I felt was best on a mission to mission basis.