A 3-1 superiority should be enough, more is better. This is quite good considering that Axemen still need a 2-1 superiority to be effective and they cost more than twice as much.
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Now to their overall effectiveness:
Quechuas keep you safe from barbarian archers. An early deluge of archers can be a huge problem in high-level games.
They allow the hardest, most decisive rush in the game. If you can disconnect their copper/iron, they are overwhelming. If an AI spawns close enough that it would normally become a problem on high levels, you essentially get a second capital for free instead.
*EDIT: They are also a fabulous unit for choking an opponent by parking them near their starting city (cities?). As this prevents an AI from hooking up strategic resources and neither archers nor warriors can displace them without excessive losses, this will totally cripple them and allow you to expand at your own pace. You can even relax the choke a little from time to time and steal a few workers that way.*
They remain effective way into the classical age as a part of a mixed force. Taking along few cheap units as instant garrisons of newly-taken cities make sense. On the way, they function as hammer-efficient throwaway units against entrenched archers: Wear them down with Quechuas, clean up with Swordsmen is often the most efficient way of taking cities. Same approach as suicide siege, only the effectiveness comes from the Quechuas' low cost rather than the ability to damage multiple units.
If there are melee defenders, simply use something else.
Building warriors and upgrading them with conquest cash is a useful trick to keep an offense going; essentially a limited form of rushbuying. In the classical age, knowing Hunting normally prevents this; not so for Incas and you also get a free promotion. You can keep doing this up to Macemen if you don't know Hunting/Engineering or up to Pikemen if you don't know *EDIT: Civil Service.*. Bit of a niche use but cool and useful.
Strongest UU in the game.
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Generally: Does it really matter all that much whether a less experienced players says 'what's so good about x, I don't get the hype?' or makes the unsupportable assertion 'x is useless!' ?
Stronger players also keep doing this, and here it leads to a lot more confusion.
, every time I have played Incans I have avoided hunting to get the most from Quenchas. 
