Frankie

Having a mixed set of MBs and CBs myself,   The CBs are a bit more forgiving in the long irons ,and the MBs create more feel in the shorter irons.  These guys usually carry 3 wedges.  The cutoff number depends on the tour player.   Check the player profiles and see what they prefer in their current setups.

Deno

All about confidence. If a player feels they hit another club from another set better than what they are hitting now they are going to switch.

I've got some fun in my set, 710 Ap2 3 iron at 4 iron length. 

5, 6, 7 in CBs and 8, 9 in MBs

48, 53, 59, 63 Vokeys

I've tweaked the lofts so that my "3iron" goes 215, 5 iron 200, 6 iron 185, 7 iron 170, 8 iron 155, 9 iron 145, PW 130

Plus...since I'm a Titleist instructor...I show case the brands! The yardages just work for me and it allows me to dial in my Vokey yardages...since that's where all the scoring is.

RC

Frankie-
There is typically a performance reason for incorporating a mixed set.  Our Tour fitters fit separately for long, mid and short irons to help the player dial in consistent yardage gaps as well as playable trajectories.  All four of the current Titleist iron models (MB, CB, AP2 and AP1) offer distinct launch and spin characteristics, which allows our players to mix sets in order to optimize performance.  You might want to check out this blog post for more on the topic, and please let us know if you have any other questions:

Titleist Irons in the Mix on Tour

All the best,
Brice Waddell
Titleist Golf Clubs Marketing

Great info on this topic. I do it myself in my own setup.