Personally and professionally, I think it's a bad idea.
We never did it in the unit during military operations. We fiddled around with it in training, but saw far more potential for disaster than positive outcome.
When you're in a fight, especially where weapons are involved, you want to control as many variables as you possibly can. The weapon you carry is one such variable, as is the ammunition you choose for it.
Different ammo in the same magazine gives you variables that are hard to keep up with. "Let's see, did I just fire all my subsonic rounds or not--or do I have the 95-grain rounds up next?"
Or, "Oh (stinky stuff)! I thought that was buckshot in the tube but it was a slug--and it went right through the bad guy and hit a child standing behind him!"
Things happen damn fast in a fight. You can't remember which round is which when they're staggered.
For the most part, everytime we went out on an operation, we carried the same handguns, same long guns, same sniper guns, same ammunition, etc. Occasionally you would change that for a particular mission, but in our line of work, not all that often. We planned our mission sequences bearing in mind the tools we had and were most comfortable and confident with.
In my law enforcement agency, staggering rounds was a firable offense--no if's, and's, or but's about it and no warning or second chances.
Jeff