It's just as simple to use a variety of other services to pay.

With any BTC payment service (including Coinbase), you also get the option to pay by QR code (if using a mobile wallet is easiest) or copy/pasting an address (if you're using a desktop/web client) which is at least as easy as redirecting to Coinbase, signing in (if you aren't already), and then redirecting back to the merchant's site.