to clarify, /castrandom would choose a random spell or ability from the list specific, then try to cast that, and that's it, if the cast fails, castrandom is already out and not doing anything

rapid successions on castrandom would eventually hit the spell that's off CD but like it has been said, it could take a few hits before reaching the one that just came out of CD

I played with five death knights and each had a castrandom with ALL of their abilities that I could use, and PvP was as simple as pressing 1 (Death's Grip round-robin) and then spamming 2 (castrandom) which would simply DEVASTATE the opponent with all kinds of unholy deadly things going at them

I've been using DoRandomToggle but it's not too useful if you want high DPS, because many times for an example my Aimed Shot is off CD and not being used for half the time of its cooldown (4 seconds out of 8 with the glyph) because of global cooldown and Steady Shot

what I would recommend is to be on the safe side, and also to get this http://www.razerzone.com/gaming-mice/razer-naga/