Doesn't matter. If you're able to sell ten times as many anvils, then you're able to sell ten times as many goblets as well.
Edit: And besides, the more rocks you request, the less other stuff you can get. This may or may not matter, but it's worth mentioning.
I think it basically boils down to the value/weight ratio, to increase the total amount of dorfbucks you can stuff in the caravan to get what they bring - and/or that you actually want. In short, it's a matter of efficiency. (On a different note, cloth/silk crafts weigh virtually nothing, making them crazy-efficient trade goods.)
Another problem is if the caravan can't CARRY enough anvils, or if you have to buy extra crap you don't care about in order to get it on there - not a huge problem, but it encourages them to bring more crap and less useful stuff next year. This is especially troublesome if they don't bring any wagons (i.e. elves). I don't think humans typically bring rocks, either, but wood works as a substitute for weight pushing. If you need it.
And again, without mood Legendary-ing, training up metalsmiths leaves a lot more near-useless junk than going for platemail. I don't know if they weigh the same (I would expect platemail to weigh less), but even mediocre armor has some use in keeping your military dying or injuring each other, if only until you have better-quality ones.
Iron isn't always really easy to get... fool that I am, I've mostly embarked in grasslands and forests so far, and the only reliable source of iron has been melting down Goblin junk. Without magma this also uses up a ton of wood/charcoal, and I've never really been able to outfit my military with full sets of iron armor, let alone steel.
On the other hand, a single anvil is a LOT less to carry to the trade depot than 100 craft items. If you keep your haulers busy and you consider their usual work a high priority, it can save a lot of time if a handful of anvils can pay for everything you're planning to buy.
But platemail can be binned. A heavy bin may take forever for a dwarf to carry to the depot, though.
But yeah, using a legendary one if you get it first can work pretty well.