You only need to use bear cannisters on the HST if you do not camp at a site that has bear boxes (they are about every 8 miles on the HST). I've done the HST each year from 2000-2006. If you can go from Crabtree Meadows to Whitney Portal in one day, you can go without a bear cannister. Most people can't, they stay at guitar lake, where there is no bear boxes, so a group of you can get by with just one bear cannister as all you need to store in it is your smelly stuff at guitar lake.
Now the advantage of a bear cannister for each is that you are not tied down to staying where there are bear boxes.
If SEKI has changed their rules since 2006, this advice is outdated then.
In 2005, we did the HST one way, 3 of us, and we only took one bear cannister for the 3 of us (not one apiece). We used it at guitar lake.