I totally see and agree with what you say in your example. the only thing I was wondering was whether or not that scenario occurs enough in the "real world" for defraggers to really be necessary on ext2. In this age of 100+ GB drives do many people ever really fill the drives over say 70% (pulled that number out of my butt). :x
I was looking around the net to find people that have asked the same question as you did, only to find one person who had written a program that would create and remove several large and small files just to see if he could fragment the drive. Apparently from his test he was able to fragment it heavily and perfomance did suffer from it. He also asked at the end of his article whether his test was real world or not. Whether he had "overdone it".
Anyway... everything I saw effectively said, yea fragmentation does occur... just not enough to really amount to anything worthwile. I myself am curious if this is really true. I don't really do that kind of stuff with my computer and have had Linux and FreeBSD systems run for long (over a year) periods with no noticeable slowdown. I'm not knowledgeable enough to know how to check for it other than when the system boots as it mounts the partitions it tells x.x% non-contigious etc...
Regardless, if you ever get around to it and discover anything either way please come back here to let us know. I for one would genuinely like to know
Jim