Q:

A/C still malfunctions after repair.  How concerned should I be? 

The A/C compressor went out on my '05 Sienna.   
I replaced the compressor but not the receiver/dryer and expansion valve. 
Now the A/C is sort of working right: It has separate front/back controls.  If both front and back is on, it works fine.   Blows nice and cold. 
However, if just front is on, it does not get cold and blows barely below cabin temp air. 
My question is: Is this a situation where not fixing it could cause further issues, or is it fine to leave it like this and deal with the minor inconvenience of having to have both the front and back on? 

A:

My experience with AC is limited to what I have been studying both in book and video form.  From everything I'm reading you really do need to replace the receiver drier when you replace the compressor.  This is due to the excess moisture allowed into the system as it's open for the repair.  Also, if the compressor was damaged causing the need to replace it, bits of the compressor may have collected in the receiver drier, and now will flow back to the compressor.  
Was a vacuum pulled on the system before recharging it?  If not, then excess moisture is still in the system and could be damaging the AC system components.  Pulling a vacuum on the system boils off the moisture inside it without needing added heat.  If this isn't done, there will be moisture left in the system, and that's bad for your AC. 

A:

Sounds like an electrical wiring sequence problem. 
As such, being in a minor system like A/C, there's probably no damage being done to the system by having it wired as it is.   The ideal, of course, would be to take out the systems and re-wire them as they should be wired.   However, you're not risking blowing something up or anything by leaving it as it is.