But a mortgage agreement in principle means nothing unless it's an agreement on your house - otherwise you still have the survey and the renegotiation to get through which is when you find you r 'mortgage in principle' buyer actually wants to agree a new, lower price.
What your suggesting, I think, is that you always want to be the end of chain vendor and that's just not realistic.
By the laws of physics, someone will always have to join the chain at another point to complete someone elses transaction.
As a vendor, I would always want a cash, or mortgage agreed buyer. Living in the real world, I accept they may not exist or want to buy my house so I will choose the best offer from the offers I receive.
And that may well be from someone who is just putting their house on the market.
Every buyer and seller has a list of requirements which are weighted with different priority.
I disagree and know from experience that if you haven't sold your house and you make an offer on another, the vendor would expect you to arrange a survey and/or begin conveyancing - they wouldn't BECAUSE they would only accept your offer in principle and wouldn't remove their property from the market.
People talk, they negotiate - they may only get one offer, they may get a hundred and need to choose the most suitable to their unique circumstances.
Yes, there are types of buyers who are genrally preferred but they bring their own big issues.
Cash buyers always want a bargain.
And do they really have the cash?
Mortgage in principle means nothing until it is tied via a survey to the house being bought and the post-survey price finally agreed.