I love the jailer role. In almost every setup a write now, if I want some kind of protecting role in the game, I use jailer over standard doc.
Jailer require there to be a tradeoff for being able to protect people, and its a tradeoff that varies based on the target player's role which brings some depth to the jailer's analysis as to who to jail and when.
When I get the jailer role, I see it primarily as a protective role, and its utility as a roleblocker as a secondary tool/mitigating factor of its primary utility. The jailer allows you to prevent the mafia from killing off an obviously pro-town or otherwise very helpful player, at the expense of nullifying that player's ability to acquire additional information via night abilities if he has any. You get the boon of keeping that useful person around, but at the expense of gimping them if they are a PR (or if they aren't, perfect! all the benefits for no loss). Using your protective ability to keep very helpful players who are likely scum targets alive should (IMO) be your primary focus as a jailer.
However, there are certainly many circumstances in which prudent use of the ability strictly to RB someone is acceptable. If you've got someone who looks scummy, but for some reason isn't an acceptable or available lynch, yet whatever power they claim to have or you have very strong reason to suspect they have poses significant threat to the town in the coming night, then obviously you would do well to block them. Same goes for if you have some very good reason to think you know who will be performing the mafia/indy/any anti-town NK or whatever, and preventing the action at the source is crucial. Just shooting in the dark trying to prevent nightkills or whatever is NOT smart jailer play. You tend to have more information readily available to you that will suggest who is likely to BE KILLED as opposed to who is likely to be doing the killing (because face it, if you had such a good idea of who THAT was, they should be being lynched, no?).
And plus, trying to indict someone with a case like "I JAILED HIM AND THERE WAS NO NK OMFG INSTA SCUM LYNCHLYNCHLYNCH" is pretty shallow and lackluster at best. A whole slew of thing could happen to prevent an NK at night, namely, mafia no shooting, kills overlapping, something like an abduction out prioritizing an NK, bulletproof getting hit, etc.
You're better off going for easily measurable and direct benefits, i.e. saving useful town players from untimely death.
