Well, if you're doing the HIIT stuff properly a lot of the time you're noodling, and when it's go time you're in zone crankin. Base training is different in that it's hours and hours in your aerobic zones which does some different stuff that may not benefit you right way, but you will notice once you start doing intervals again come build time. Done right, for me it seems to add repeatability and depth (the ability to go about as hard deep into a ride). You also don't really cut out all intensity, you just like sorta dial it back. Like instead of doing vo2 work I'll do 4-5x20 tempo during a 5hr z2 ride, like with one interval per hour, and the rest in z2. It's less polarized, but tempo 4.5hrs deep can feel pretty bad. I'm not going to say 15hrs isn't a lot of work, because it is, but I seem to get the most out of base when I'm doing 20+hrs a week. It's something that works best when you have a lot of time to pedal and have optimized day to day recovery.
Anyway, the answer is, it depends. How long are your races, what are your long-term goals, how long have you been riding, can you handle the monotony of long rides mentally, are you good at keeping on top of nutrition, etc.