As much as I love the convenience of just pulling a bucket off the shelf, we've always mixed 90% of our standard or most used colors. Have to agree with Peter that spotty availability/discontinuation of colors + lack of control of the mix further sours the idea.
The real killer for purchasing pre-mix colors, if you have a PC system, is that your in-house mixes might cost less than pre-mix, provided you are mixing somewhat efficiently and don't have really high labor costs. At least that's how the math worked out for me.
Wish WFX would adopt the QCM setup where they had pre-mix WOW inks and HO low bleeds. Keep it simple and offer a very basic range of quality colors in both. Then make sure it's stocked and available in gallons everywhere.
Another thing I would like to see is ink companies tweaking the formulas on pre-mix colors to perfectly suit the color- adding something to help keep yellows from picking up like mad, for instance. As it is, the colors you mix with your PC system are exactly what you get when you order the pre-mix, aside from the super-fluoros and whites of course.