When is it worthwhile to aim for visits from Nobles?
Generally I'd look to go for Nobles in one of the following situations:


There are 2 or more Nobles with matching colours. E.g. if there's a Noble that requires 4 green and 4 red, and also a Noble that requires 3 green, 3 red and 3 blue then it might be worth picking up the 11 cards (mostly from Level 1) required. If there's also a Noble that requires 4 green and 4 blue then that's 9 points for 12 cards which looks very attractive.
There are Level 3 cards which match colours on a Noble. E.g. if there's a 4 point card that needs 3 red, 6 green and 3 blue and also a Noble that needs 4 green and 4 blue then I might go for the Noble and the 4-point card.
In a 2-player game, there are only 4 tokens of each colour so it can be much harder to buy Level 3 cards. Nobles can be an easier option for 2-player games.


A few other thoughts and observations.


unlike cards, Nobles cannot be reserved. On level one there are 8 cards of each colour, so really the only way someone can stop you getting a Noble is if they claim it for themselves first.
it's possible to pick up a large number of Level 1 cards very quickly. Once you have a few cards you can pick up Level 1 cards for 1 or 2 tokens, or even for free.
buying a card costs a turn, a visit from a Noble does not. So you potentially save a turn which can be crucial.
if scores are tied at the end of the game, number of cards bought is used as a tie breaker. If you went for Nobles and the other player didn't then you'll almost certainly lose. This does happen occasionally, so it's worth bearing in mind.