Originally Posted by ConnFlyer

It doesn't really matter as it has no real impact on your earning of SkyMiles, MQM's or anything else.

As to why DL is doing it, number conservation. DL is starting to run out of four digit flight numbers. The CO/NW codeshare is eating up lots of numbers, then add in the existing codeshares with AF,AZ,KE,JM,AM,etc. Then add in all the new RJ flights DL is constantly adding (new cities like HVN,TVC,EUG,BPT,etc) and even more numbers get eaten. DL also needs room for when new SkyTeam codeshares start (KLM is coming and eventually Aeroflot).