ModPlug has a great feature where you can do a search and replace, which I think would be terribly useful. I just wrote a whole song using a C-4 of one instrument and now I’d like to change it to an F-3 of a different instrument with an effect- the search and replace would be perfect.
… i never need this … i know the structure of my song - every note every effect … don’t know could this useful?!
I would have had this feature lots of time when tracking drums (which is a process you may noticed takes a lot of time in my composition 
), when you are using single-instrument-multi-samples drumkits.
Sometimes you may want to substitute a drum component with another (f.e.: each F-4 should become a G-4), and it is a pain when you realize this after having finished 40 patterns…!! 
Some others could find this useful for some chord variations.
Another great use of this could be to change an effect value globally: say you are using a VSTi which has note gliding ranging one octave, then you suddenly realize that you want to substitute it with another one which has a raning of one tone. Probably you may want to s&r certain 91 xxxx
with others.
Obviously, the “search & replace” box should let you define the scope of the action, and let you confirm or not each substitution.
I imagine it this way:
two track rows, one matching the row to search, another one matching the new row to be written.
The only problem with this one is:
what about multicolumn stuff? 
Probably a search in multicolumns checkbox should solve this.
Anyway I can live without this feature. 
understand … ok … thinking about drum-mappings …
may the search function could be combined with a multiple (block-)selection feature > u can search somthing multiple results in a pattern will be selected > or u can use a free defineable selection mask (e.g. every second line on one column) … after this u can use advanced edit to manipulate the selection
anyway … multiple selections (holding Cntrl) could be very useful!
by the way i don’t know why the word “Block” is used for simply (pattern)SELECTIONS … 