Would anyone have bought the bonds without this clause?

I'm not sure, but keeping in mind that these bonds were issued at a time when Argentina was considered extremely risky to lend to, I'd imagine this was more than just an oversight on the part of some lawmaker or beaurocrat. If this clause makes it harder for Argentina to reneg on their promises (as they have done repeatedly in the past) that strikes me as crucial.

It's quite telling that they structured the deal in NY and not Argentina. I don't know why a country would agree to that unless they knew it was the only way foreign investors would consider lending them money.