What Rob said sounds ok.. But I think the only issue here is if you come in and tank for 50% of the run, some dps gear falls from the first boss and you are tanking spec.. You tanked it. Should you roll on it over those who started off with a dps spec? I'd feel .. odd.. about doing that myself.. I'd not do it.
Lets say I show up as a tank in the *new* instance Uldr and we kill first boss.. I'm not sure I'd want to roll against mike for a healing item that drops. He could equip and use it right there to help us move forward.
The other way: ppl could decide what spec they are going to roll on before the raid and stick with that (regardless if you change up
during the run for a fight or two) If you sign on in GC as a tank, you stick with tanking gear through the entire run, even if you change to dps/healing during, for a few fights. Just like me MTing in naxx 10, but doing some healing for a few fights. The RL should use you
primarily for the role you signed up for.
Typically, in the end, ppl end up with a lot of stuff for multiple specs anyways. So you are probably going to get much of the gear you want for both specs. Like in naxx, most of our group has main/off spec epic gear. So the question is: Who should get what loot first? I think the answer is depends on which spec you come with. Which spec the RL wants you to identify with for a given raid.
So:
Loot
1. Should 2nd-spec be considered a main spec?
(Not if I were the RL for given run, unless you start a given run with a 2nd - spec)
2. If not, should 2nd-spec be considered superior to off-spec?
(I don't understand this one. You mean 1st-spec superior to off?
W/e spec you sign up with in CG for a raid should be your superior spec, at least during a new encounter until it's down. For naxx10, ppl could probably spec into their off-spec, provided they have gear good enough, though it may not be as good as their main spec stuff.)
Expectations
1. Should raid leaders expect their players to have more than one spec as soon as they can afford it?
(Meh, I would not if I was a RL although see answer to 2.)
2. Should raid leaders expect their players to spec in two different roles?
(I would not, although those who do bring more flexability to the table and by that reason, may be more sought after, since they could change up before or during a run.)
2b. Is it rude to the other raid players to have two identical roles?
This depends on the attitude of that player. It could be rude if a person who has 2 roles they play 50/50 get greedy with loot or their place in a run. But it could be very helpful also.
Ultimately we can settle, prevent a lot of issues by simply dealing with things before the team heads in. So ppl have a clear understanding of things and don't feel like they are being treated unjustly. Talk to the RL about which role/spec you want to play and see if that's ok. Just let that be your identity for that run. And Ask yourself if gear is = across both specs, so you can contribute equally. It might not be a huge issue for easier content, but the new stuff it will be.