I have a couple situations that I think might be conducive to using a sink-tip fly line.
I've never used one. I've use full-sinking lines a couple of times, and find them more suitable for trolling deeper water from some sort of water craft than for casting. Not that I can't cast them. But it certainly isn't something I'd want to do for hours at a time.

So perhaps a sink TIP is the way to go. Here's my situations, please offer your solutions:
1. Large Spring-time Crappies in 7'-20' of water. Lots of rocky snags within 20' of shore. Wish to reach 50'+ on a cast. A fairly slow presentation is best, but prefer a "horizontal" fly path/pattern.
Thinking about this a little more...perhaps a WFF line with a REALLY LONG tippet section at the end of the leader might work...with a strike indicator above? I'm not convinced that is the answer either, though, and might be difficult to cast, especially since wind will probably be an issue.

2. Fishing a good-sized river that varies from 4' deep in late Summer to 8'+ in the Springtime. Flow rates can be 3'/sec or more. Rocky snags on the bottom are common, water clarity is usually 24" or less. Want to keep close to the bottom, but allow the fly to ride up over the tops of the rocks.
Target species will be white bass, carp, walleye, freshwater drum, channel catfish and possibly flatheads.