Buying a set, and wearing said set together are two totally different things. I know quite a few things in the gold shop that I want to be more than one colour of the same thing. I usually buy one out of a whole set because I don't usually have much gold on hand.
I'm always kicking myself for seeing a new gold shop item that I love, and then buying it in every color.
I won't always buy the shoes, AND the coats, AND the hats, etc., but, for example, the Raven tails in I think Barton Boutique, I bought all of them, and have only used the red/ black one.
If it's an item that I find later on, while trying to jazz up an avi, I'll usually behave and only buy the one color that I need. (Forehead jewels, for example.)
I buy whichever items I feel will come in handy when I design new avis, whether at the moment or in the future.
I never use more than one set item at a time though. I like the challenge of creating avis with no sets.
But having the items on hand when I'm playing around with items & poses is very helpful.
I prefer to mix and match. It is very rare that I'd get a whole set, and now that I think about it I can't say I actually own a "complete" set of items... I don't think I actually do, at least not a set of more than two items anyway. I'll have to look. confused Usually I'll buy only what I think is useful, and for non-store multipose items I'll usually only get two of that item max. You know, because sets lack creativity and all that jazz. Though I keep in mind that owning a set of items and using that set of items on one avatar are two totally different things. After all, different parts of a set can be useful for different applications.
I will admit I think there is only one item I wouldn't mind getting a couple more of to complete the outfit set... but that's only because I really like that item.