In terms of colour fixing, I'd say 3+ colours is where it starts to add up. Being able to crack an
Arid Mesa to go and find
Godless shrine in my mardu Aristocrats deck is super useful.
Fetches also have a lot of additional utility though.
They're great in any deck that cares about landfall triggers, regardless of the number of colours they are running.
They're also great anywhere that runs
brainstorm/
scroll rack type effects. Being able to shuffle on command can be very useful in the right place. For example, I run a couple in my Yuriko deck almost entirely for this effect. It rarely needs colour fixing, but the ability to shuffle still makes them worthwhile.
Not to mention, they can fuel things like delerium, delve, threshold, revolt etc.
Basically, in 3+ colours or anywhere that the additional utility is part of the gameplan, I think they're worth it.