That's true and not true at the same time. For weapons, legendary and set pieces are trash. Complete trash. If you can't get a weapon that does 7-800 plus base dmg you will have a hard time in inferno, and almost none of the legendaries hit those numbers.
For armor, on the other hand, set items and legendaries are essential. The reason is that they often have stats that you normally would not find in other items, and the stats they have are exceptional. An example of this is Tal Rasha's chestpiece. It gives a +attack speed bonus, and afaik is the only chest piece that does that. It also has at least +50 resist all and exceptionally high armor.
Another example is Zun's boots (journey maybe?). They are also +attack speed. Then there are the set bonuses. Some of them are complete trash (Tal Rasha's for instance), and some of them are amazing. It's not uncommon to find a set w/ 2 piece bonus of +50 resist all.
Also, glad to see we have another player in inferno. The group I play with has been farming the butcher pretty heavily to gear up. We have a pretty solid system:
Set your quest to "The Cursed Hold"
1) Highland crossing, kill 2-3 blues in the fields. Find the Cave of the Moon Clan. Should have 2 more blues in there. Go kill them, second floor has a yellow chest. Make sure you have 5 stacks before you open.
2) Switch to gold find/MF gear if you have it
3) Damp Cellar (area before the first waypoint). Often has a treasure goblin.
4) Cemetary of the Forsaken. Blitz through this. Should be 5 elites and one quest that should earn you a ton of gold.
5) Switch back to regular gear.
6) Go kill the warden. 2-3 rares
7) Go kill the butcher. 2-3 rares
Rinse and repeat. An afternoon of this will earn you a massive amount of rares (a few of which will be valuable) and gold. Now, if you think that's lame/cheesy, the dudes I'm playing with have also figured out how to farm Inferno Ponies (as in the highest end area in the game.)
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Another note for inferno is that you ABSOLUTELY HAVE TO get your resists and armor up. I am rocking around 400 resists and 4k armor, and I've just started seriously gearing. Life on hit is also incredibly important. Most of the peopel that are complaining about inferno just have lousy gear, or have focused all of their gearing towards DPS instead of defense. Offensively you will be fine with about 20-30k dps (ignore the 50k + builds, you will die constantly). Some fights take longer, but you'll beat them. Defensively you want your overall damage mitigation (resist * armor) up to ~90% (~65 on both). Your health should be ~20-40k at least. Right now my vitality is wayyyyy too low for A2 (27k life), but I'm hoping to bring it up soon.
Right now I can solo the butcher, and he isn't able to one shot me. Act 2 is exponentially worse, but I'm pretty sure I'll be ok on it once I get my resists and life up more. In fairness I have been playing with a gold find gem in my helm. I should probably switch that out....
People just need to accept that inferno will require a LOT of time to get geared for. The AH is your friend, and until you get well geared for inferno, gold find is also your friend.