I'm nearing the end of my fishing quest; just the Blood Lobster and Tunnel Crab left to catch now... and I'd say for crustaceans the Suit is absolutely worth while. Each time the Traps complete a *bob* the chance of a rarer crab etc goes up, as does the chance the trap will be destroyed. As it's multiplicative then, it also multiplies the effectiveness of the bait applied to the trap... combine that with the fishing suit which will also be multiplied in turn and you'll see a much greater chance of catching the Legendaries whilst equipped.
For Legendary Fish however, it's hard to quantify. It's not noticeably better, if you've only got 100 charges of some of the rarer bait (and some have never dropped for me at all, despite over 300 recorded fishing quests now) you'll likely lose them all, suit or not. However the odds of catching a Legendary are so low (Kylie is reporting around 3000 fish for each, which is roughly the odds I've seen too) that even a little help will be desirable, especially for areas with intense and dangerous spawn.
Wanda and Phineas on Europa have donated Charybdis bait to me to host public events a few times; until Publish 80 the suit was broken however, and no one has offered bait since, so I can't tell you if that encounter is improved with the suit... however the maths don't indicate it would be noticeably improving; it's 25% chance above GM fishing, the same as Osiredon... and 1% from the suit isn't going to move that much. Again however, the ability to get the materials is so ridiculously low, anything which nudges the chance of successfully summoning him is worth it. Of all the 450+ submissions to the Fishing Quest document stickied here, I'm the only one so far to get the Oracle, and none of us ever got Charybdis bait... so yes, too damn right I wear the Suit when trying to pull him up! (And it's a good job I did, as that helped prove the suit was broken still, and got it fixed). Osiredon however is, as far as I know, just a straight chance based on Fishing skill, you won't need the Suit for him at all.
Smelly Bait? I've only ever had it for the Rare fish, never Legendary, and Rare are pretty friendly with your hook... I'd say it is more effective, but I couldn't quantify it in practical experience. If you've got the bait, may as well Suit up I guess!
