So I know that this page has been up fora while, but I still see comments as recently as a month old that still seem to be confused about how Arcane scaling works. Playing an arcane/jack-of-all-trades as my main, I felt I would try to compile what I've learned via digging on other sits and here in one place.
WEAPON SCALING: Yes, all weapons have arcane scaling, but they only benefit from it if they already have natural elemental damage, like the Burial Blade or the Tonitrus. Otherwise, this indicates how the weapon will scale IF you put an elemental gem in it (which will remove the physical scaling). The Tonitrus, for example, will always scale a bit with Arcane because it naturally has bolt damage. The Threaded Cane, despite having B scaling in arcane, gets no benefit until you put an elemental gem in it. At that point, it does pure elemental damage and it will scale with ONLY Arcane, making other physical stats above the minimum requirement useless.
ITEM DISCOVERY: IT increases item discovery up to 50, just like it says on the wiki.
DIMINISHING RETURNS: So here is where Arcane gets weird. For scaling on weapon damage, it works just like anything else. You see a soft cap at 25, and a much harder one at 50. The damage from Hunter Tools, however, does not appear to have a soft cap. I'm capping my own level at 150 with 60 arcane, so I can't speak to the upper limits of the stats, but I have read elsewhere that the damage on these tools will continue to scale very well up to Arcane 70 or 80. This will NOT affect the damage on weapons, however. Past 50, they stop benefiting from scaling to any appreciable degree. Taking your arcane past that limit is strictly to boost the damage of your Hunter's Tools.