Jimmy, It's basically a personal preference. I can name a variety of reasons why I don't like pre-made manifolds. I'm not sure you want to hear them all. What I've found to be my some of my best tools being an irrigation tech are, observaton and anticipation. I will name one reason why I don't like that Spears manifold. It doesn't matter that I've never seen one in person. Look at all the unions. Each one has the possibility of leaking someday. Much more than a simple male adapter screwed into the inlet port. I also think Boots is right.
If I can't fix it, it's broken!