Is it a good thing that Spotify doesn't guarantee that they won't leak your data? So you're saying that it would be a good thing for them to make it easy for people to steal your payment information?
You can't be sure that they actually delete your data but you /can/ delete your account.
My point being that to guarantee that would obviously be irresponsible - since there is no such thing as 100% data security. It's way better, in my opinion: "you probably shouldn't put anything too sensitive here - we have been hacked and will get hacked again". Obviously they will try their best to secure their systems either way, this because the biggest cost is the "user reaction" (users leaving the service or alike) when getting hacked.
You can't be sure that they actually delete your data but you /can/ delete your account.