Some training plans are based on the observation that a. a lot of people have limited time to train b. the races amateurs do are typically pretty short and c. going long and (relatively) slow doesn't help a lot beyond basic conditioning when it comes time to race for an hour or less.

In short: no intervals aren't equivalent to longer rides, but depending on what you want to accomplish, they might be better. Let's say you had planned to go out for a couple of hours at a moderate pace, because this is what you enjoy. If a storm came through and you did some quality intervals on the trainer instead, you'd probably actually gain more fitness, since I assume you're pretty well acclimated to riding for a couple hrs in one go. The same can't necessarily be said of replacing a fast, 4 hour group ride with intervals (those are higher intensity and volume) or even trying to do a fast century (huge endurance component).

If you want to win crits, intensity will be key. If you want to do RAAM, then you'll probably want a greater training volume. If you just want to stay quick and enjoy cycling, you'll probably do a little of both. If you just want to bike a lot, and don't care about speed at all, then just do whatever you want.

It gets easier to train when you have something to train for.