I'm pretty sure they use the hard plastic line due to it's durability. It will survive much longer than the rubber lines do. The lines on my car are a couple years older than yours and still holding up nicely, but then again I don't have a cat. You might see if you can get any of the MB hard line to replace your aged sections, just use the rubber lines as splices where the ends meet.
You probably could use the black rubber stuff to repair whole sections, but it'll break down quicker, plus the hard lines are somewhat color coded. On mine I have yellow, black and opaque, some cars also have green and brown as well, to help ID the various systems.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.