...I won't mention names
Yup, that would be me.
No, I don't like to see wiring that is intended for one purpose being use for something unrelated.
Cat5 is a communications cable and it gets confusing if it is used for doorbell, thermostat etc. This aside from the fact that the conductors are 24ga and can't handle much current. If you parallel them it gets really confusing to somebody who didn't install it. My philosophy is to use communications cables for communications and the proper cable for other uses. This way you can see at a glance what's what.
As to using the second pair on quad cable for power, I really don't have a problem with it (and the use is related too). Note this is 22ga quad (2 pair) that I'm talking about not 24ga 4 pair cat3 or cat5.
If you look at the cord that is supplied with the transformer it probably is 22ga. Granted it is only about 6 feet long but the power consumption isn't great either.
I can't see the objection to having the 12VDC and the T/R under the same jacket. We know that even low voltage AC will not be a hum problem because this is done routinely. Certainly then DC will not be a problem.
If you are worried about crosses, 12VDC would not even begin to bother the phone if applied across the T/R which normally works with 48VDC.
I'm only pointing this out as one possible way to do this which may be more convenient than a separate wire sometimes. If you are more comfortable with separate wiring that's fine.