Last time I jumped a fellow my low revs didn't charge his battery whatsoever. Had to hold my revs in the 1800 range to charge his battery somewhat. The jumper cables I had were not capable of carrying full starting current from my battery to his. That's why they are called "booster cables" rather than jumpers (on the packaging).

Reason you don't want to connect post to post is that there will be a spark generated on making the final (4th) connection. You don't want that spark to be close to the top of the battery where hydrogen gas from charging is generated. The spark could propagate into the space above the cells and blow up the battery. (or so I'm told) Having a battery blow up in your face would be a very bad thing.