I think jeff88 means closed loop, this is when the PCM uses the O2 sensor to start adjusting fuel trims and happens within a minuite or two of starting. 02 sensors use heaters to get up to temp fast, you can observe this using a scan tool.
There are a few reasons it's not good, if you don't reach operating temps it won't burn off water vapour in the crank case, it also won't burn of hydrocarbon vapour in The crank case which when it's mixed with water vapour becomes corrosive. There is more blow by the piston rings when cold so that's how these combustion byproducts get in there. You are also heat cycling the engine a lot, expand and contract, expand and contract, wash, rinse, repeat. Over time that stress breaks down fluids, metals, plastics etc. you will foul plugs because you will be running a richer mixture and they cannot burn off the carbon deposits etc, etc.
I would buy something with highway miles over something that has been driven over a million short trips any day.
Honestly you might not see any difference in the short term, but if you do this day in and day out for YEARS you will see something. I had a buddy who took off before the engine oil even began to circulate in his truck, after a few years the thing was dead, no compression.
I live a 10 minuite walk from work, I could drive every day but I don't for this exact reason.
