Here's an extreme example where this makes a huge difference:

I have a dir tree with large numbers of small gps log files, in CSV 
format, and I want to use ruby -a (autosplit) to work with them.

With RUBYOPT=rgem-fallback (or of course RUBYOPT=''):

$ time find . -type f -exec ruby -F, -ane '$F' {} \;
RUBYOPT='' find . -type f -exec ruby -F, -ane '$F' {} \;  2.06s user 
1.67s system 39% cpu 9.431 total

With RUBYOPT=rubygems:

$ time find . -type f -exec ruby -F, -ane '$F' {} \;
find . -type f -exec ruby -F, -ane '$F' {} \;  219.02s user 61.52s 
system 93% cpu 4:59.26 total

Of course, awk would probably be even faster, but ...

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407