If you're going for guard skillups and you're subbing nin, you're retarded, end of story. No monk with a brain subs nin until 74+ anyways when they start cannoning in ToAU, so that's not even a valid point. I tanked a good portion(at least 10% of every fight) of leveling my monk up to 75, a year before monks had to sub nin to everthing and I still barely even saw a guard let alone a skillup. When I go for guard skillups, I sub whm; but only because I do not have pld or rdm leveled.
Your chance of guarding is lower than your chance of parrying, that means you WILL guard less and have fewer chances to get a skillup. Therefore, it's more work to level it up and therefore tougher. When you're soloing though it's 100% on you, and that's what I'm talking about for skillups. No shit you skillup parrying faster when you're tanking, stop acting like it's some kind of revelation when it's not even relevant to what I'm talking about.
There's a bunch of people with capped parrying, but almost no one with capped guard; obviously that means they're of equal difficulty. :wacko:
You don't have to rest as long with signet, you still have to rest more often without utsusemi.