There will always be someone falling for scams. No amount of safeguards will protect them if they do zero due diligence and the scammer is persistent enough. The checkmark isn't an end-all-be-all, but it's another small step someone can use to verify without too much hassle.

also, I just noticed "they" is ambiguous here. I meant "the twitter staff giving checkmarks". At least I hope they do some basic check before handing out a checkmark to an obvious impersonator.