Not everyone using Bitcoin is concerned with anonymity, and those who are use Tor anyway, so their DNS queries are already anonymized. DNS query results get cached, too, so not every query is going to be visible to the person running the authoritative DNS server for the domain in question. Even without Tor, if you use a popular public DNS server like 8.8.8.8, 4.2.2.2, etc., then there's a fair chance your queries will be returned from that server's cache anyway. I don't see how this is any more of a threat than running a Bitcoin client without Tor.
It's because you're crossing a security boundary with a bugged address. Basically, if you posted your 1xxxx address on a torified website and you sent to it, no other information but that address has been passed on. When you use a vanity name backed with txt record, you're putting a permanent bug in someone's client - since now it's a.b.c.d that needs to be looked up to be sent. At any time their client leaves the security domain you have the chance of leaking IP addresses.
Not to mention having to wedge all of dnssec into the reference client in order to validate the signature.