3rd Gen Artisan Cave

This is a cave in Emerald only where you can find wild Smeargle in. You can also find Vitamins here. This is pretty much the only way to capture Smeargle in the main series games in Generation III without relying on Pokemon Colosseum.

Did you bother with Artisan Cave? I do sometimes, since I do want Smeargle, just in case I want to transfer one up to Generation IV in future playthroughs...

As you implied, the only reason to go there was to capture wild Smeargle in Gen 3. The Vitamins are the icing on the cake here.

I do wish it was a bit more fleshed out, with some Trainers and a sidequest of some sort though.

I was disappointed when I first played through it and there were only smeargles. Also the Altering Cave. But I guess if you had friends to trade with it didn't bother those people.

I never had anyone to trade with, and when I was younger my parents were a lot thriftier than they are now - whilst my Xbox360 got replaced twice, I had to sell my GBA on the cheap to get the money for a new DS game - so I don't even have a GBA but instead use a DSLite. So it has always irritated me to have these barely-used sections of the map. I'm probably more irritated I didn't hold on to my GBA though. I keep getting tempted to buy a used one, but given it'd cost at least twice what I got for mine 10 years earlier I'd feel a bit of a mug.
Smeagle confused me greatly when I was younger too, I didn't understand why it had only the one move that it had stolen from my Pokémon. I understood move tutors, but not that if I trained it up it could learn Sketch again, it seemed a waste of Heart Scales for a quite sucky Pokémon. Now my inner perfectionist enjoys getting another filled entry in the Pokédex at least!