Well, I see it like this. First of all, everything in manga, which is the original and truest version of a manga/anime pair barring a few cases, is always to be considered canon, no matter what the anime does. Secondly, OF COURSE Kakashi Gaiden is important. It explains a thousand things that people who only watch the anime and don't read the manga are probably still incredibly confused about (I don't know, I don't pay attention to the anime anymore). It explains a) where Kakashi got his Sharingan and b) more depth of why he's always visiting the KIA Memorial thing. But most importantly, it gives Kakashi more backstory, and backstory is ALWAYS significant to a character's place in the plot. And beyond that, there are a number of different deeper things I've noticed. Take, for instance, the relationship between Obito and Kakashi. It is almost exactly the same as was Naruto and Sasuke's, and for that matter, was the same with Jiraiya and Orochimaru too.
Kakashi Gaiden is very important, and gods know why they didn't include it. They probably thought it would've made a better special, movie, or possibly even spin-off (thank the gods that didn't happen, can't stand anime spin-offs) than if they were to include it in the anime itself, then decided against it (or not... *imagines possibilities*), but were too late to include it before Shippuden stopped. And now they're just waiting for a good opportunity to slip in a long flashback for Kakashi which is basically Kakashi Gaiden, heck, maybe even as a filler.