Right you say guards will take more shots given it is a LI and it will be more with a 2-3 than man to man. By if you think about it, in man to man the bigs get lots of chances, which end in a higher percentage of makes. However, guards don't seem to have as good of a percentage (some may but lots don't) so the 2-3 defense forces more work on the guards which is actually better when playing against a LI. So if you know you are playing against a team that has stacked bigs then why not force the guards to do more work?