SPF's great benefit is protecting your users against phishing attacks. Common phishing targets such as Paypal, eBay and Gmail, as well as banks and other finance organisations have very well managed SPF records. The onus for ensuring SPF records are correct falls on the sender, not the recipient. If someone inputs an SPF record for their server, then sends an e-mail via a different server you're doing exactly as requested by blocking the message. Problems of misconfigured SPF records will ease once more organizations implement it aggressively so that administrators will not be able to assume their mail is flowing correctly when delivery is initially successful to a few lax servers.