Maybe people whose letters of offer have expired should use it as an opportunity to see if they can do better.
The mortgage market is moving very quickly in this country and there may well be a better offer waiting out there for you.
It is not an inconvenience. If I told you when you're about to buy something in a shop that you could get it for thousands cheaper down the street, you'd hardly say "Well I'm here now and it would just be so much hassle to walk down the road....oh, and, this shop assistant has been so nice that I couldn't possibly offend him by not buying from him!"
Of course, if you would then go ahead and execute the first letter of offer you get.