Hey Mike.

For my liking, an ultra-wide lens is essential for landscapes, but it's worth remembering that even telephoto lenses are used in landscape images.

I like a wide vista, and the ability to get close to a foreground subject of interest and make it larger than life due to the inherent perspective distortion an ultra-wide focal length provides.

Certainly explore your landscape photography with your 17-50, and if you're finding that the framing isn't wide enough to suit your style or preference, then it's time to look into an ultra-wide lens.

It can be all too easy to load up on gear to preemptively solve problems before they're actually problems, so give your photography a go with what you have, and with a bit of experience under your belt you'll be in a better position to determine if an ultra-wide lens is what you want and/or need.