I hope you've got luck with your cover-photo! It's definitely not a cheap lens and yes some primes will offer the same or even better IQ at the same settings. But it does offer a nice range, IS and fast USM. Things that primes in that range don't necessarily have. But there are definitely some options in the prime-department at this range. Check some other threads if you need more info.
I personally have thought about buying the Canon 28mm f2.8 for the times I need a lightweight kit. It does seem to perform reasonably good wide open and it's cheap. Second hand it's a real bargain.