In my opinion they are of absolutely no use. I have no moral objection to the death penalty, and I do think people like Picton/Bernardo have forfeited their right to exist in society.
However, I have a pragmatic problem with the death penalty. The problem is not so much that Picton/Bernardo have a use or deserve to live, the problem is that in most cases there is no perfect way in which to guarantee 100% who is guilty of a crime. Unless that ever actually happens, its probably best to avoid the death penalty (and any potential mistakes that it might entail.)
If they are caught in the act of committing a crime for example, or if it's obvious from the circumstances of the arrest. An example of the latter would be if you arrest them where they live and their car and/or apartment contains an arsenal. Or property lifted from the victims of erotic nature such as I would have expected with Picton. When the Pictons, Berkowitzes or Homulkas of the world are "tried" the issue is generally their psychological state. While if caught alive we have no choice in having a psychological exegesis on someone of no discernible value I think we should unofficially try to work towards minimizing these situations.
Why do we need to administer justice in a 'system' ? Maybe because it's more consistent than just having individual magistrates decide what to do every time ?
You are inside the system you say ? Well, you must realize then that you're proposing a complete tear-down of the system and you must have some larger ideas as to why/how this should be done so rather than ask someone with a basic understanding as myself, why don't you explain your rationale and proposal ?
My proposal (see above) is that if someone is literally caught red-handed their arrest becomes a bit rough. But as for the Court system there aren't words to describe how creaky it is. Judges and, where applicable, shuffle in at 9:30 a.m. at the earliest and get down to work, if you're lucky, around 10:00 a.m. Ditto the start after a two-hour lunch break. Before you jump in and say they should work longer hours the problem is that the Judge has other things going on and emergency orders to sign. Sometimes there's an emergency hearing that erupts and delays the schedule further.
During the actual trial there are sidebars to resolve evidentiary issues. Sometimes one or more of the many players (12 jurors, the Judge, about two lawyers per side and the witness) need to attend to "personal needs." And this is for trials that aren't theater or spectacles, like Manson.
Holding a multi-witness trial is rarely quick or efficient.