yep agree on the one bottle to get through a std length xc race (~1.5hrs). if it is hot, neck another bottle immediately before the start, or put a 2nd bottle on the bike, or have a spot trailside to pick a 2nd bottle up half way thru the race.
i am surprised that the qlders on here aren't dead against camelbaks, unless it is absolutely necessary. i am assuming that you guys have to often deal with temps closer to 30+ more often than we do in NSW. a hydration pack, no matter how compact, is gonna make you heat up and sweat more in hot weather.
in most instances*, i'd only take a hydration pack if i the water bottle carrying capacity is not enough. i am no pro and have major difficulties trying to handle a bottle on rougher twistier courses, but i still manage, and am happy to deal with a compromise of slowing down a bit while drinking.
i am a bit envious of those that can take 2 full size bottles on their bikes. even on a hardtail, the smaller size frames (that i am usually looking at) only fit one bottle.
*having said all this, i sometimes take a hydration pack on endurance-type events even if i know i could make it to the designated water stops with just a bottle - i have dealt with water stop queues, and prefer to avoid.
as stated above, for multi-lap races, where you can leave a stocked esky by the trailside - water bottles all the way.
another reason to take a hydration pack, would be if you want to be able to have two different drink types on the bike, and only have one bottle cage. i.e. one bottle for carbs/electrolyte, a hydration pack for pure water.