-No frequent training is not "just for fun". I think what MI_KB'er was trying to say is there is more than one path to strength. You can train the CNS for strength (frequent, low rep-high tension "practice"), hypertrophy (muscle size, using something akin to the Bear routine), or a mix of the two.
-The reason GTG is not mentioned in PttP is because PttP was written before the Naked Warrior.....nuff said. Plus, how would Pavel or the Party make money if one book had it all?
-I think the basic reason there are only 2 sets a day (and mentioned in the book), is that unless you live/work in a gym it's pretty hard to deadlift and sidepress throughout the day. Whereas with bodyweight exercises, you can whip those out anytime since you always have your bodyweight to work with. Yes you could technically GtG the DL/SP, but most people wouldn't last longterm on a routine like this unless they are pro lifters.