Building base is mostly accomplished by doing high volumes at an aerobic/endurance type effort, the key being as much volume as your time allows without the need for significant recovery between workouts. Consistency that a diet of harder efforts may disrupt. The benifit is change in metobolic processing (your body stores and processes glycogen more efficiently, your body learns to process calories while riding, etc.). Additionally, the significant volume of aerobic work makes your aerobic system significantly more efficient (many aspects of your cardio system become better like the stroke rate of your heart).
So, is a 20min effort at TT pace aerobic or not. What about a 5min efforts? What about studies that show that metabolic efficiency adaptations occurring at high intensities (HIT) ? How does lots of Long Slow Distance work help cardiac output when that sort of effort range works more cardiac eccentric hypertrophy and very little concentric hypertrophy?
Some may say you won't be able to complete the efforts as effectively (i.e your general fitness won't allow you to get the same quality workout as if you had done a block or two of base). Others would say you risk injury because your body has not been properly prepared. I personally agree with all of this, but my big thing is that I feel there are much better ways to spend your time training in the off-season. Your AWC and VO2 can be fully developed in a matter of 6-8 weeks, why write these "checks" from your training account before hand when it can be better spend developing your FTP, which takes much more time to develop. Now I am not saying AWC and VO2 won't improve your FTP, I just don't think it is the most effective way to do it.
So "base" is fitness, and the rest is really all speculation and conjecture with no science behind it? Got it.