Interesting discussion (well, parts of it anyway, once you sort through the usual MTBR gibberish) on the FB forum on MTBR over the last week or so. Click here if you want to read through it all instead of just hearing what I have to say.
What stands out for me is that one poster posits that a "race" bike can, and should, be built lighter (and can be less durable) than a "trail" bike. I think this is a bit of a false dichotomy in mountain biking - we're not racing Indycars here, after all. Comparing a hardtail XC bike to a 6" travel "trail" bike isn't the point, because the long travel bike is intended for a totally different type of terrain. Assuming you're riding the same kind of trails both while racing and while just riding (which is what 95% of us do) I would argue that race bikes need to be burlier, not lighter, than their standard non-racing brothers.
Here's what I know - I've raced as a pro here in Colorado (a really crappy one, but then again I don't do any training) for almost a decade. In that time I've had an awful lot of bikes, some of them advertised as "race specific geometry" high-speed podium finishers, others as just regular old mountain bikes, and everything in between. For a good portion of that time, I've built my own bikes and raced them. Here's what I've learned:
-Race bikes take WAY, way more abuse than bikes that aren't raced. Don't believe me? Go scroll through some Craigslist ads for used dirtbikes sometime and see how many times you see the phrase "never raced" (as an aside, you'll also see "woman owned", which is a neat phrase meaning "Despite her objections, I purchased a low-end dirtbike for my wife/girlfriend and she refuses to ride it.") When _I_ race, I tend to take stupid lines, go faster than I should, take blind corners at high speeds, and generally behave as if my cerebrospinal fluid has been replaced with a cocktail of mephamphetamines and Red Bull. Of course, this doesn't mean that I actually do well in the aforementioned races, but I certainly put my bike through the wringer. Hence I submit that "race" bikes have far harder lives than those which are never raced. I submit a photo of the start of a Boulder short track race (grand prize: PBR) as proof of my theory:
-Race _geometry_ is actually the opposite of what most people think. You do not want a bike with a low trail number for racing, at least in the Rockies, because by the time you get to the downhill (and the bike handling) you are way too tired to be fighting to keep the bike going in a straight line at speed. Predictability and stability are key for race bikes, even if that means sacrificing some low-speed nimbleness. You're always faster if you don't crash and aren't afraid to let her rip on the straight stuff. This goes double for enduro racing, where you're going to be completely wasted both mentally and physically by the end of the race. Of course, every course and local area is different, but my point is that super-twitchy geometry does not always a race bike make.
-Frame weight is virtually unnoticeable. Sure, an extra pound of weight won't make you faster, but it also isn't going to change where you finish unless you're within a few seconds of the next guy or gal. A pound of frame, for an average guy, is 1/2 of 1% of the total weight of bike/rider/gear. Do you want to ride a heavy bike? No, not really. But a frame that handles predictably and fits well is WAY more important than a few hundred grams of weight. The frame is the last place to try to save weight, IMO. Most other components merely need to be durable - as long as your seatpost doesn't break, it doesn't matter, so get the lightest one you can. But if you have a frame that fits or handles poorly, you're not going to be fast no matter how light it is.
-Unless you're getting your frames for free, or you're independently wealthy, a durable frame is a better choice than an ultralight one. I think I broke 2 different Homegrown frames when I rode for Schwinn - and I never could have afforded replacements if they weren't free. There's a reason the really fancy bikes you see at a race are mostly ridden by Sport 45-49 guys who are tax attorneys by day. Look at the pro field and you'll see some fancy bikes ridden by the top guys (who are increasingly rare these days) and then a whole bunch of *random crap* ridden by the rest, most of whom are really, really fast. These guys are well aware that they could buy a "nicer" bike, but they also know that it won't make them any faster, so they save their money for entering races and/or not working as much so that they'll have more time to train. If you aren't rich, and you're buying a new bike every year you're being penny wise and pound foolish, as far as race results go. Less bike and more training will make a huge difference, so save your $5k (assuming you like your current bike), take some unpaid vacation days to train, and hire a coach while you're at it.
Bottom line, I guess, is that for me, a bike that is a good bike for riding trails with your friends is *also* a good bike for racing. There is no such thing as a "race" bike in my book. Most of us "race" our buddies up and down certain trail sections on pretty much every ride, and try to go as fast as we safely can all the time - hence the bike is really doing the same duty it would in a race. Ultralite disposable frames (how many Trek 9.9s are still around, only about 5 years after being the most popular race bike on the planet?) are a poor investment if speed is what you're after - invest your money and time in your fitness and skills, not fancier equipment.