A few things that may bear consideration:
1) The inflator must be immediately accessable at all times, and in a consistent location to enable you to locate it without having to see it. Typically, the length is only sufficient to be retained in such a position and still reach the owner's mouth, drysuit inflator, etc. By using a combination reg/inflator here, you either need to increase the length of hose to accomodate gas sharing with this reg, or lose the benefit of the retained position.
2) A standard inflator mechanism can and is used as a tertiary backup regulator, by breathing off of it while depressing both the inflator and vent buttons simultaneously. (such a situation might entail negotiating a tight restriction while donating gas, and rolling off the left post) A combination reg/inflator unit will either be intended to replace the traditional backup, in which case you are losing an entire second stage, or to supplement it, in which case you are unnecessarily complicating the system by adding failure probability to your buoyancy control device.
3) If using the combination unit to replace the traditional backup second stage, you have now lost a second which could otherwise be swapped out with another stage if malfunctioning, opened and cleared underwater, etc., with something unique that can not be easily cleared or swapped with anything.
4) It is a poor idea to constrain a regulator to a device which is used for other purposes, when the necessary functional location and necessary range of movement of the two devices are not the same.
5) Contingency response to a stuck inflator mechanism (mechanical failure, frozen or fouled) can include immediately disconnecting the LP hose from the offending device. With a combination unit, you either disconnect the reg as well in this case, or if equipped with a dual fitting for the purpose, have now complicated things in the exact area you need to get to in an emergency to perform the disconnect.
6) The above scenario raises the somewhat obvious point of the failure probabilities of the regulator and the inflator combining to result in a less reliable device than either of the two independent components.
7) A standard second stage, when employed as a backup regulator, is restrained beneath the owner's chin in a position which enables him/her to switch to it hands-free. I doubt that you could arrange the combination unit in an identical fashion effectively, but even if this were possible, you now have precluded the ordinary function or the inflator by keeping it in this position.
8) In the aforementioned position, while primarily intended as a backup for the owner, a diver in need of gas can still go for this reg immediately, pull it free of the restraint, and breathe from it. Such a diver, who may quite likely be panicked, is not apt to excercise extreme caution in where they place their fingers when grabbing the device. A combination inflator/regulator unit in this scenario places the buoyancy controls in a position that allows such an individual to operate them accidentally while trying to obtain gas.
-Sean
