It seems that /castrandom is generally taking precedence over /castsequence because it logically chooses abilities which are not on cool down (or so I believe). Now that haste has become such a standard part of the game, and there are so many proc abilities/cooldowns, /castsequence is becoming increasingly difficult to use effectively. This weekend (don't get to play much if at all during the week) I'm going to try swapping over to /castrandom because I'm tired of my toons dropping out of DPS during a fight until I notice them and then hold up for a second. I swear on my mage she's just stood there for 10 seconds while I'm spamming the big-bang button because haste/procs put her out of sequence and she was standing there waiting for a cooldown.