Using it to attack Bitcoin won't reap much benefits especially when some BIPs would probably shift it to a quantum resistant algorithm fairly quickly.
we could soft fork to add a new quantum-resistant signature type fairly quickly, but at the same time, the amount of bitcoins stolen from legacy addresses could have a catastrophic inflationary effect on the market.
since moving one's coins into quantum-resistant addresses is voluntary, millions of coins would likely remain unmoved. those would be stolen and circulated back into the economy.
that was the thinking behind theymos' comments here, which were
not well received:
https://www.reddit.com/r/Bitcoin/comments/4isxjr/petition_to_protect_satoshis_coins/d30we6f/This issue has been discussed for several years. I think that the very-rough consensus is that old coins should be destroyed before they are stolen to prevent disastrous monetary inflation. People joined Bitcoin with the understanding that coins would be permanently lost at some low rate, leading to long-term monetary deflation. Allowing lost coins to be recovered violates this assumption, and is a systemic security issue.
it's a sticky subject. theft is wrong, but so too is willfully ignoring all users' expectations of controlled bitcoin inflation. lost coins were supposed to be a donation to the rest of us---at least that's what satoshi said---not recirculated into the supply.