The first actual question

EGR in diesels.

Do some engines use it? If not why?
You would think it would help because the increased heat would help combustion, and there is always air in the diesel exhaust (at least on stock diesels). so wouldn't it add power and fuel economy?

Yes.

EGR reduces combustion heat. The dilution of the intake charge with exhaust reduces heat. Its main focus (on gas and diesel) is to lower combustion temps in an effort to reduce NOx emissions.

The higher compression of diesel engines typically makes a lot more NOx than gas, so EGR is almost a necessity. Newer diesel EGR are intercooled to prevent adding extra heat which further reduces NOx.

I think all deisels produced after about 1980 have EGR.

yes, and unfortunately, on some vehilces the egr cause more problems --they get clogged and don't self clean, throwing a CEL on the dash with some stupid codes. ugh!

egr is a disaster if you are looking for performance disable that one. CAT didn't put egr on their new OTR truck engines because of the problems instead they paid the EPA a fine for every motor with out and have less problems.

CAT has the ACERT system now that meets 2004 EPA standards. Cummins, Detroit, International-Harvester... everybody else basically went to a cooled EGR system to meet 2004 EPA emissions standards