On occasions I've worried that this was a waste of time and effort. But ultimately I think that building multiple sites has been worth it. This is because websites are akin to investments. And as any good financial advisor will tell you, you should have several of them running (I think they call it "diversifying your portfolio").
Basically, if you build multiple websites, you'll find that some bear traffic fruit while others do not. You can then concentrate on those ones that are working. In the meantime, you can figure out why the others aren't doing so well and then solve the problem at your leisure.
You learn a lot about SEO doing things this way, too. For example, I have one blog that's in the online marketing field (a blog about blogging, mainly). Being in such a saturated niche it took ages to get even a few hits per day from the search engines. One of my political blogs, on the other hand, was much easier to draw traffic to. That's because the subject wasn't nearly as competitive, and I was targeting very specific, often topical keywords.
Having gained those insights, I can then apply them to the online marketing blog. For example, I can write more posts that are quite specific and topical -- less evergreen -- to increase the number of people arriving via search engines.
If I hadn't built multiple sites and instead focused entirely on that marketing blog, I may well have built up the traffic slowly. But I wouldn't have had the comparison to reflect upon, and therefore wouldn't have realized that there were other approaches available to me.