I fish full sinking lines both in lakes and in the salt.
In still water you may want to count down to a depth and then keep your line at that same depth for as long as possible during your retrieve.
You can't do that as well with a sink-tip. Sink-tips pull your line toward the surface the moment you begin your retrieve.

In the salt, besides the reasoning above, when casting in high wind, the skinny, heavy full sink line cuts through the wind far far easier than a thicker floating will.
And many folks like an intermediate line in the surf as it rides below the chop making it easier to stay in touch with your fly