Actually you don't need DIYCE "to break GCD", it's game issue with GCD skills (or in that case 1sec CD), probably same works for M/S shot... In PvE it doesn't matter you fire X skills in 1sec, than you will have X sec CD for any in-game action. Same happens with P/S ice blade before 3s CD and same happens with any no CD skill, just most of them have restricted "energy" pool, so you do not see rouge with shadowstab spam etc.
As for DIYCE. First of all it's not so perfect. Actually using DIYCE means over 20% less skills, than you can use normally pressing one key, so I can manually "cast" CC 118-121 times during 2min and only 88-98 using DIYCE. The problem is that if your character have any rotation, you have to use one skill, than another, check boss debuffs etc, DIYCE do it all for you and do not make mistakes - EVER. So usually you can gain about 10-20% of DPS using DIYCE, but there would be fights where you actually will do less DPS. Besides making DIYCE do everything for you is really stupid.
Let's say, that some boss cast spell X, that have to be interrupted. Whole PT is using DIYCE, so first cast everyone interrupt, second well if you are lucky, there is some guy, that have another skill to interrupt, third time you are supper lucky and another one, forth get some lottery ticket CDs from first time wears off, 5-th sorry dead. Same with mana/hp pot, using any burns and doing whole boss tactic.
Don't get me wrong I still thing DIYCE is great tool making game much simpler, usually buffing DPS etc. The problem is that first off all you still have to know instance and most of all your char, to make own combo and use it properly (when you simply copy/paste combo, than it usually wouldn't work for you). The only problem I can see is that you have to know lua enough to make own combo, but it's simply changes skill names from defaults combos in addon. I use DIYCE myself, but if I will just stay there my DPS would be still 10-20% lower than it is now and I will fail more often.... Without DIYCE I would be probably at current DPS lvl, maybe with little more fails (than know and probably same, than using only DYICE) in first month, as I would have to adjust.