I like Spike alot too hehe. What a great character! I don't mind his prominence at all.
But seriously, in response to the original question, I don't think Spike is the focus of the show at all. Every scene he has is created by Angel's character directly or indirectly. There is a need for a character like Spike: Someone to contrast Angel's endless brooding. If anything, Spike reinforces Angel's character with every line he speaks. It gives us a great opportunity to reflect that Angel isn't just a 'Vampire with a soul', but is so much more complex than that description. We have Spike filling the role as well, so we we are forced to look deeper at Angel to see what makes him different than Spike, and thereby find a truer essence of his character.
I have always felt it isn't what the characters do that make us love them, it's how well we know and understand those characters. Spike gives us the contrast we need to understand Angel better.