I went through the B-29's at Aberdeen around 1981 as a representative for the Imperial War Museum, Duxford to acquire parts for their B-29 It's Hawg Wild. I removed a lot of parts like Turret system components, bomb rack equipment, hydraulic panel etc which are now on Hawg Wild.
The B-29's at Aberdeen were out in a tall sharp sticker bush type of weed about chest high and were virtually inaccessible on foot. I was driven to the aircraft on an all terrain tug. The aircraft were pretty rough at that time with a lot of corrosion visible throughout. I remember seeing inter-granular and heavy surface corrosion in many areas especially blowing through the longerons at the bomb bay door hinge boxes.
There were still lots of equipment on board but some were more stripped than others. One aircraft was off by itself and was slated for a museum but I can't recall which museum.
A few years later I remember seeing an ad that indicated the remaining airframes were offered at government auction. I found out after the fact and I never heard where they went. I feared that they may have gone to scrap.
As far as why they were there can be explained why B-29's were at several of the major weapons test centers namely China Lake, Socorro, NM and Aberdeen. I believe that the technology gained by the Russians from the B-29's that they captured in WWII went into their post war aircraft. The construction of the B-29 was revolutionary and that same basic design is used in pressurized aircraft being built today. I think that the B-29's were perfect for weapons testing because if you figured out what was the best weapon to destroy a B-29 it would be equally effective on the Russian aircraft of the day. This is purely my speculation as I have no documentation to back it up. They used a lot of B-29's at the three weapons test centers. A lot of the survivors today were lucky to escape.
Here is an old post on the subject of surviving B-29's. Gary took some great shots of three China Lake survivors brought out by Kermit which are at Aerotraders storage facility in Borrego Springs:
http://www.warbirdinformationexchange.org/phpBB2/viewtopic.php?t=17557&postdays=0&postorder=asc&start=0Here is one of Gary's awesome shots:

Wonderful and interesting photos. So is all these parts saved?