i know it's needed on earlier thinkpads, at least, to allow pcmcia
cards to work.  i remember a frustrating attempt to get a modem card
going on the butterfly thinkpad that was resolved by a similar change
(once i'd finally noticed that a Linux pcmcia configuration file
handled the thinkpad specially and started its equivalent scan at
0xD0000, presumably because the thinkpad has got something else there).