I don't know. I just have no real connection to Spike. He's almost like a background pony to me. Sure he has a few lines, but it's mostly comedy relief and nothing really that important. I'll admit he saved the Crystal Empire. But it's rare that he does anything important or worthwhile for the plot. It's really all about the mane 6. But that's okay with me. I don't dislike Spike, but it wouldn't really matter to me if he wasn't in the show anymore.
I would not consider him a main character on the same level as the mane 6, but pretty much the closest to a main character outside the mane 6. One may argue that the role of the closest to mane 6 in terms of "maincharacterdom" might go to Applebloom or Sweetie Bell, but personally I think Spike does take a more active role.
Apart from the aforementioned saving of the Christal empire (and the games there), we sometimes get him in his role of pulling the emergency break when Twilight is having her over the top moments (lesson zero), we have him with his own issues (Dragonquest, Secret of my Excess, Owls well that ends well, Just for Sidekicks, Power Pones and Spike at your Service) and we also get some character development, especially with regard to his attitude towards Rarity (comparing episodes like "A dog and pony show" to "Inspiration manifestation").
One may think of him what one will, but in my opinion his contributions are quite important and Twilight would have lacked something without her number one assistant.
Last but not least he occasionally gives some of the clishee "guys" comments in a show where there are but very, very few male characters none of which other than Spike play a very frequent role (Discord in six episodes total I think, and Shining Armor (much less active than Discord) als in six if I didn't count wrong).
Twilight Sparkle: Dear Princess Celestia,
Today I learned something amazing. Everypony everywhere has a special magical connection with her friends, maybe even before she's met them. If you're feeling lonely and you're still searching for your true friends, just look up in the sky. Who knows? Maybe you and your future best friends are all looking at the same rainbow.
Spike: Gross! When did you get so cheesy?
Twilight Sparkle: Just write it, Spike.
Given Spike's unawareness of context I found that one quite hilarious.
