The RB211 is a fantastic engine - outshines the CF6 and the JT9/PW4000 family hands down - but the reason that these aircraft would be worth less than those equipped with the PW4000s (worth the most); CF6 (next highest) or JT9Ds is simple - there are a lot fewer of them around.
This is the same problem that the DC10-40 (which had the JT9Ds installed) had - all other DC10s had CF6 power. It's a question of commonality, as well as training, support, etc.