Frankly, I think marketing is much more important than technology for success. And I say that as a software developer. It doesn't matter how good your product is if nobody knows about it. It doesn't matter if it's better than the competitor's if the competitor is better at telling people why they should use their product.
I currently work at a SaaS business. Our app and marketing sites are one and the same. Some of that is for historical reasons, but I believe that for us it's an advantage. We grow steadily in part because the product team works more closely with the marketing team than with anyone else in the company. Exposure, conversion, and retention are baked into every feature and every decision we make. We could never do that nearly as effectively with an outsourced marketing department.
One caveat: we are a mass market B2C company. I wouldn't be surprised to find that there's a sliding scale for how closely marketing and product need to be integrated depending on how high-touch the sales process is. An enterprise B2C where there's no way to try the product at all without a custom-negotiated contract has no reason to integrate the marketing site with the product.
That said, I'm always skeptical about outsourcing marketing entirely. See the first point above.