I think whether you work as a freelancer or hired employee both decisions, to work in your niche only (or at least almost) or to provide a broader range of services, are completely valid, also marketing wise.

I certainly agree that, once you specialize in a niche (let’s say wordpress design) you’ll get loads of referrals and customers, which want exactly what you do, wordpress design.

But many clients are also happy if they can put all their web/design-related needs in one hand, instead of many.
A client (especially smaller enterprises) doesn’t want to go to a logo designer to get his logo designed, then to another designer for his stationary, next stop web designer followed by a web developer, maybe add a illustrator and animator on the way, etc.

I agree that it is rather difficult for a single freelancer to provide all these services to a high standard, but it is not uncommon, nor a bad decision, to provide logo and web site design in combination with front-end development.