I think the combo system is a good idea and pretty well implemented at least for the classes I play. I dont see it as a replacement for battle regimen and we need something that is more focussed for use in a party.

With the exception of Archer, having people dart all over the battlefield in a party just to position themselves for their combo looks retarded imo.

Coupled with the fact that some mobs have powerful directional attacks that you really wouldnt wanna catch the back end of if your combo timing put you in the line of fire.

so basically...

Combo system = great for solo
Battle Regimen (or whatever its replacement will be called) = great for party (assuming it is implemented well)