*One* purpose of proxies is indeed to deal with firewall restrictions.
*Another* perfectly valid purpose of proxies is anonymity. You may not agree with the use case (I generally don't), but it happens to be one. Just because you believe the vast majority of "anonymous proxies" are hackers doesn't justify removing behaviour for the valid use cases. A discussion about how many people use proxies responsibly is as irrelevant as a discussion about how many people use /fserve for legal content-- they're both features that should be properly implemented with the responsible users in mind. Anonymity happens to be a use case for some proxy users, and it's hardly your place to make any judgement on their behalf.
The rest was answered quite succinctly by Sat.
PS. it seems you don't quite understand what DNS Information Leak is and why it can be an issue, so
here.
