@Drakon, the Enclave on the ENCLAVE used T-51b as training armour or simply as an alternative to forklift trucks.
It just shows the T-51b was not so obsolete or that the Enclave manufacturing was not fast enough to completely phase out the (now) obsolete T-51b armor were the Mk II was used on mainland operations that carried more danger that being on the Oil Rig, the Advanced Power Armor was in all aspects more capable that the T-51b, the Advanced Power Armor(s) given a +4 strength, the T-51b a +3 strength so there was no point in using the T-51b for "lifting" over the Advanced Power Armor outside availability.
That was another of Fallout 3 problems, they downgraded the Advanced Power Armor Mk.II because they make the Enclave troops the "Goblins" were in FO2 taking on a Enclave Patrol WITHOUT Power Armor was near suicide and also because they made the T-51b a unique reward as the old Mk.II was now the most common available armor they made the T-51b simply better to justify jumping all those hoops, that almost carried over to FO:NV were the T-51b helmet still have the idiotic +1 Cha and the Remnants helmet have the -1 Cha, not to say if the extra 3 points of DT justify the weaker Radiation Resistance and the weaker item HP.
I know the T-51b are Brotherhood maintained suits as the Remnants must have let their suits rust away but still ... one suit can easy be found as the other either requires knowing were it is and you have to fight a lot of Deathclaws for it AND you get a T-51b suit as well for the trouble AND you still have to find its helmet or manage to trigger a side quest with Arcade near the end of the game were you gain one.