"As of patch 5.18, I prefer [Warrior] as it has solid stats for Nocturne and is not as risky as a devourer enchant if your team is getting punished in the early game and you can't find time to farm. If the game is not going well early on, I would opt for a cinderhulk instead." - ugapa, Master Nocturne Jungler, in his guide on Nocturne jungle on lolking (http://www.lolking.net/guides/255384).
Obviously you may have better luck with devourer on Nocturne at lower elos, and if you are having success, don't let other people bring you down.
There are a few reasons people have been telling you not to build devourer. First, Nocturne doesn't really have the tools to solo carry games. He's much more suited to helping your lanes snowball. Nocturne's early ganks can be devastating. Landing a Q does a significant amount of damage and allows you to chase people down. If you can finish your E on enemies, the movement speed and fear almost guarantees a kill or flash. W is great for early ganks, when there's only one ability that can save your target. Nocturne's ult is great at finishing low health enemies or joining a skirmish that started before you got to the lane.
The Warrior enchant helps all of this. Your autoattacks, Q, and R scale on AD and armor pen. The cooldown reduction means your ult is available more often and you might be able to get a second Q off to finish a low health enemy. Devourer on the other hand, doesn't really help you with early ganks. Sure, 50% attack speed is nice, but what does it actually mean in an early gank? 1 or 2 more autoattacks? If you're lucky it gives you a second proc of your passive. Plus, 30+ on-hit magic damage should always be less than the extra damage you get from 45 ad and 10 armor pen.
Once you do get to the late game, you'll have Sated Devourer, which is really nice with your passive, but what else do you have? Let's compare Nocturne's kit to Master Yi, a champion who most would agree should build Devourer.
Passive: The passives are pretty similar actually: hit people to hit them more. Nocturne's gives a small amount of sustain, while Master Yi's has a slight edge with the double hit.
Q: Nocturne's Q has good base damage, good scaling, and gives you AD while you're on it. It's great for early ganks, teamfights, and even poke wars. Master Yi's Q also does decent damage in a small area, but it also gives him an instant-speed dash instead of movement speed and an invulnerability frame. Invulnerability is huge in the late game. If you can kill your enemies in 3 hits and they can kill you in 3 hits, getting in 2 hits before they can get in 1 is amazing. Plus, Master Yi's Q scales with attack speed because of the reset mechanic.
W: Nocturne's W is great in early ganks, when you can absorb some cc and proceed with your easy gank. In the late game, it becomes much more difficult to shield the correct spell. Even if you do, there's probably something else that they can throw at you. Master Yi's W gives him healing and damage reduction. In the late game, this can be used as a stall to let the cd on Q refresh so that Yi can go back in.
E: Nocturne's E is, again, great in early ganks. Single target CC that gives you movement speed is ridiculous. Late game, it will be much harder to keep the tether and even if you fear someone, there are usually 4 other people around. Master Yi's E gives him 5 seconds of true damage on autoattacks. Not only does this scale with attack speed, like his Q, it's best in the late game, when people have enough armor and mr to make devourer not hurt so much.
R: This is probably the most obvious difference. Nocturne's R is great for assassinating single targets, not for diving into a 1v5 fight where the vision denial won't even matter. Master Yi's R has a passive and active that both scream late game carry. Resets are always extremely strong. Add to that bonus attack speed, movement speed, and immunity to slows and you have a formula for strong melee adc.
Second, besides his passive, Nocturne's kit doesn't scale with attack speed. It scales with AD, Armor Pen, and Cooldown Reduction, which happens to be exactly what Warrior has.
Third, (and this builds on the first point) Devourer encourages Nocturne to farm the jungle instead of doing early ganks. Large monsters are easier to kill than champions. Scuttle crab vision is nice, but doesn't snowball your lanes.