I have a few bait runner reels and quite like them. A Penn liver liner 560 and a Shimano D 8000. I generally use the Penn as first preference probably due to the smaller size. I have also used the OC 6000 and Thunnus 4000. I have been considering purchasing a Thunnus but the CI4 seem to deflect more at the base when a fish is on. I am not sure I like it.
I like having using the bait runner function when feeding a floating bait out into a burley trail. I like them for downrigging and trolling. In all of these instances it’s just easier having that slight resistance rather than freespool.
Another thing I like is when flicking the bait runner on when baiting the hook or having just boated a fish it allows you to remove extra line in a controlled way and also reduces the chances of a crazy fish impaling you on a hook.
Funnily enough I never use the function to allow the fish to run with the bait. The few good snapper I have caught floating baits down when it hits I just turn the handle to turn it off and strike. Let’s not talk about my jewfish record!!
To answer your questions.
Livebaiting yes but don’t use the function much as I generally have other lines out and keep the lively pretty contained. Live baiting under a balloon definately use the function drifting it out.
Braid or mono. Braid I fish from a boat and never use mono.
Soft plastics no not really. No reason why you couldn’t but for that style of Fishing I’d use a nice Stradic or similar which is much lighter in a similar sized reel.
Are you fishing land based or boat?