To the original poster:
There is no such thing as too vigorous a boil, but you will need to calibrate your equipment. For instance: my strike water is always at 15 gallons, my mash water gets adjusted depending on how much grain goes into the mashtun, but it's usually a bit thinner than normal being that the mash is constantly running between 2 pumps, a heat exchanger, the hot liquor tank, and back to the mashtun. My sparge water also differs, but I always know because I've calibrated my system that I loose 1 gallon of water per hour in steam and 1 gallon in trube. I will also loose 3 gallons out of my hot liquor tank from transferring between pumps and each vessel. That means in a brew day I loose 6-8 gallons of water. To make a 10 gallon batch I start boiling with 13 gallons of wort, and for a 20 gallon batch I boil 17 gallons of wort. I do not make 5 gallon batches unless it's a pilot brew, for the folks at more beer informed me that my system would drop down to 65% efficiency if not at least a 10 gallon batch was made. In short, get to know your system and take the time to do the math to learn what amount of water is needed each time you brew. Calibrate, calibrate, calibrate. Not only should you calibrate for liquid loss, but also for heat loss. I know my strike water will loose 7* going from the hot liquor tank through the heat exchanger, through the pump, and into the mashtun. I am also quite aware that my digital controller will keep my strike and mash water within 1* of my specified target. With this knowledge I can set my digital hot liquor tank at 180* and know that the burner will kick on before the temp drops below that of a sustainable mash temp. All of these things are crucial in brewing a consistent batch of beer time after time.
To the second question, the one about the need to boil the wort, a lot of chemical reactions take place during a 90 min boil that can't be achieved any other way. Also the most important of all reasons is O2 depletion and sanitation. In fact I don't think I've ever bought an extract kit or a beginners book that did not give specific directions to boil for 60 minutes. I think the old guy on YouTube is the only one I've seen recommend not doing a 60 min. Boil, and he at least does a 10 min to sterilize. To this day I've never seen his name published in the BJCP winnings section, so I seriously question his techniques.
