The above link will take you to a set of videos recorded by the City of Richmond (Virginia) Fire Department. It demonstrates the necessity of utilizing a 2 1/2 during standpipe operations.
While this is some great stuff, I think it also illustrates a few more things, most importantly, value of the smooth bore nozzle. If you have been reading the blog at all you already know that I am a big 2 1/2 fan and I feel it is underutilized as I pointed out here http://averagejakeff.wordpress.com/2010/06/09/hose-line-selection/ .
While I do belive that the 1 3/4 is a great tool, it is not the end all and be all, there are fires that require 2 1/2 inch handlines and when they occur you have to recognize it. Failure to do so will resemble “Pickett’s Charge”, it will be brave but will be poorly planned and ineffective.