When I would designate pitchers in these roles, they would end up pitching about ten innings per season. So I have not used them at all since my first couple of seasons.
It would be interesting to designate every pitcher in your bullpen as a LH or RH specialist, so they'd have to be used, and then see what IP looked like for them at the end of the season. In theory they'd at least have the platoon edge on the first hitter they faced. But right now my team is competitive so I don't want to risk conducting the experiment! Maybe when I am in a rebuild phase I'll try it for fifty games to see what happens.