When would I use Leyline Phantom?
You've pretty much summed it up yourself. While you probably would not put Leyline Phantom in a constructed deck, it saw a fair amount of play in limited, since it both has a pretty big body for the cost and has various synergies with a few other cards in the same limited environment.

I would not spend to much time trying to figure out other ways to use the card; since the card is just not very powerful. I'm sure there are plenty of other cards that combo with Leyline Phantom, but for most of those there are lots of other cards that combo even better (e.g. the "gating" creatures from Invasion block).