Randi is great when he works, but I agree with Angel that often times he's too much of a pain in the ass to use, which is why mine has been on the bench for the last like 4 or 5 trials to come out in JP. Some reasons:
His initial damage is non-elemental, so he's not good for burst or OTK setups
If he ever does anything other than chain every single round, his imbues fall off, dropping his damage
If he dies, or gets dispelled (either by the boss or because you are removing an imperil from your team) he loses his imbues, dropping his damage
He is elemental locked for specific chaining schools:
HE = Fire, Holy, Dark
QH = Water
PD = Earth, Wind
Sometimes those elements are fine, but a lot of times you want to use a different element, which he can't do.
Now, he has a lot of upside too. If a fight lasts more than one round, and he can dps uninterrupted, he does stupidly high damage, and that's before you consider he has 125-200% innate killers for like half the game's races.
I agree with Angel completely when he says Randi is sick on paper, but for the last like 5-7 trials or so in JP there's some mechanic or another that makes Randi just not that good for the specific fight, which is why he's been on the bench.
Some examples:
Alexander -- you have to dispel yourself often, removing Randi's dark imbue
Europa reborn -- you have to use non-elemental damage, which Randi can't do
Al Pherg -- physical in general are bad on this fight (not specific to Randi)
Watcher Welker -- another anti-physical fight (not specific to Randi)