The packs on the -800/900 push out a constant temperature air which is a little cooler than crew and passengers would normally find comfortable.
Sounds like an inefficient way of doing it. Yes, the output of the packs going into the mix manifold on some aircraft is quite cold. This air has to mix with the existing hot air in the cabin to provide a comfortable temperature.
However, I've noticed the individual zone trim air valve positions on some aircraft types (not sure about the 737), and one will usually be closed. You don't want to mix hot trim air with cold pack air if it is not necessary. It would be like turning on your home airconditioning to too cold, then switching on an electric fire.
"0.00" is a completely closed trim air valve (no additional heat added to a zone). "1.00" is fully open (max trim heat applied to a zone).