I've always had natural ability as a pure endurance rider, and as such the long, slow base thing has never appealed to me as a training strategy (since it's already my strength), but I think there's more to my skepticism about long, slow base than personal preference. Low-intensity training does three things very well--burn fat, build capillary density, and improve spin. For most everything else, higher intensity training causes much greater physiological adaptation per unit time. Translated into english, slow base makes you a cyclist--reshapes your muscles, improves pedaling technique, trims body fat, vascularizes your muscles; and since the recovery time from riding slow is short, it's possible to put in massive hours--again great for the new cyclist. Once you're already a cyclist, slow base confers a much smaller benefit, and IMO should be deemphasized.
"So what do we do all winter after the beer runs out?" Personally, I switch to "the only whisky you can drink for breakfast" (according to my partner from TR last year, at right), but when I'm good and tanked enough to be able to brave the 15 degree temps, I do ride "base", and by no means is that plugging along at a fast walk for 5 hours at a time.
The success of any training activity for a particular ability is related to how well that training tests the limits of the ability, and naturally, the harder you go the more abilities you test. An endurance MTB racer is concerned primarily with two abilities: aerobic endurance and muscle endurance, where aerobic endurance is cardiovascular fitness and muscle endurance is cardiovascular fitness plus strength. (Strength is important factor in the latter because pushing muscles closer to their force limit breaks them down faster.) Spinning long hours at a pace that isn't at the limit of your aerobic power output stresses neither of these abilities.
So dump base altogether and do intervals till you puke? Though a 2007(?) issue of Bicycling suggested just that, or at least the I2Puke part, there is a reason to ride somewhat slow, and it's all about physiology. So far as I have read, the one physiological adaptation that simply takes time is the improvement of capillary density in your muscles (I'm assuming you already have that sculpted cyclist body here...). When muscle tissue is constantly in demand of oxygen and energy, the blood slowly erodes new channels to service that tissue, but like water eroding stone, it doesn't happen very fast. The capillaries also go away if you don't need them. If you were to ride as hard as you could all the time, your muscles would never be able to recover enough between rides for you to put sufficient hours on the bike for maximizing your vascularization.
Instead of dropping base altogether, pick it up a little. By riding your base miles near your aerobic threshold you challenge your aerobic system without significantly increasing recovery time (studies have shown that both fatigue and recovery time spike as soon as you cross over your AT), and if you're already a trained cyclist you'll have no trouble keeping this pace up for 4 hours at a time with some practice. For clydesdales, focus on duration. For us skinny guys, it won't hurt to go a little shorter and venture over your AT into the hills for building up those muscles. In most athletes, your hart rate at your AT is about 20-30 bpm below what it is at your lactate threshold, which is usually defined as the pace you can hold for the last 20min of a 30 min time trial.
For you voyeuristic folks here's what I'm doing (Eric is on a very similar plan). If you subscribe to my blog reading philosophy, you may choose to stop reading now:
Since returning home a month ago I've been riding pretty much pure "Base", or "BS" as it was called in our original plan layout. I tend to respond well to crunching, so I've been riding at AT for as long as time allows fri-sun, leaving most of the week open. As one of my biggest weaknesses is force, I've been trying to hitting the weights 2x/week (mon-wed), working both the upper and lower body. I keep reps high on the lower body (25), while doing fewer reps (10-12) and more weight with the upper body.
Right now, while strength training is of high importance, the riding during the week is light, generally consisting of easy off-road rides 2x week, time allowing. After about 6 weeks on the weights, weight room visits will decrease to 1x/week and the reps will drop for half the sets to 5. My philosophy behind the rep decrease is that power lifting improves muscle recruitment and coordination, which I lack. The second weight day will be replaced with hill repeats at high intensity as the start of a "build phase".
Once sprint practice and racing start happening in earnest, the weights will get dropped altogether with the hope that the intensity in everything else will retain the strength gained in the gym.
If you want to know even MORE gory details, you can view my ride log here (it will live forever more in the right sidebar). I've gotten over my guilt about the fact I never fill out the weekly goals, and my embarrasment that Eric refuses to use it at all (as a monk of the anti-datarian order, it offends his religious sensibilities), to bare all against a backdrop of google spreadsheets. Feel free to heckle me when I'm not riding enough. I'd do it for you.