They make the new revision because it's smaller(cheaper to make and consumes less power). It does cost more than their competitor, but not that much more - It's likely that over time they may be able to sell the micro cards cheaper than the pro duos. I think they should have kept the old model Memory stick pro duo also, but it didn't happen. All memory card formats are continuously upgraded and improved over the years, so I don't think you can say Sonys the only one not using old formats.
I don't think Sony was trying to sell to current PSP owners really all that much, so they didn't factor in using the old memory stick, they chose to use the newer smaller one, I don't really see why you'd get another PSP if you already had the old model especially at $250. The old psp fits in my pocket fine most of the time.
I assume there's a degree of this in the title what i get that's almost to say 'see Sony does it too, so MS isn't doing anything that wrong' If you want to know expensive proprietry memory and why MS gets reamed on this lately - look at this (off amazon):
120gb 360 hard drive - $134.50
320gb PS3 compatible Hitachi hard drive - $59 ($75 Difference and 2.67x more storage)
