After reading a diary entry about noexit=* Tags a few days ago I check out my area and...
I started to clean up my area using Osmose (which recently became available in Germany and really is an awesome QA tool!) and it's slowly getting better - have to wait for Osmose to update what I already fixed.
Many people seem to put it on ways, which is bad for routing as I found out when fixing some stuff for leitstellenspiel.de. Many tags also just seem to be very old and somehow just ended up in the wrong places by splitting streets and others are there even thought there is a foot way at the end of the road.
Use the noexit=yes tag only on the very last node Node at the end of a highway=* or on the last way Way to indicate when doubtful that the impossibility to travel further by any transport mode is perfectly normal, due to otherwise existing road layout. * http://wiki.openstreetmap.org/wiki/Key:noexit