You can actually Queue oGCDs, if you press an oGCD more than half way through your casts, it will go off immediately after the casts finishes. The reason classes can still macro oGCDs is because even then if you're say a brd, mch, or a healer, then you can only get one skill off before your gcd is back up anyways unless you have a proc/swiftcasts. So its fine if the oGCD doesn't go off immediately as long as it doesn't clip the next GCD, which most 1 skill oGCD macros won't do.