The reason many rescues won't home to people who are out all day is because many rescue dogs come in with separation anxiety already, usually because the previous owner has left them all day and/or has not bothered to train the dog that it might need to be alone sometimes. Separation anxiety can be resolved but it takes many weeks of gradual and dedicated acclimatising.
Being alone causes most dogs at least low-level stress and in some the stress can be extreme. There have been a number of recent studies where the behaviour of dogs home alone has been observed by remote camera and before-and-after medical tests have shown marked changes in cortisol levels and other indicators of stress. These were pets, not lab animals. Some, of course, didn't give a damn but you (and/or the rescue) have to find the one dog in maybe twenty or thirty who genuinely does 'sleep all day' and doesn't worry.
To the less aware dog owner or the casual observer the dog that barks and howls all day, destroys things or toilets indoors is just a nuisance but these are outward signs of a stressed, unhappy dog.
A decent rescue organisation would not be 'rescuing' a dog by sending it out to a home where it would be alone all day; that's just moving the problem sideways while waiting for the dog to be brought back in, possibly more unsettled. A dog without separation anxiety may well develop it and it's the job of the rescue organisation to protect it from that.
'I have to work and he just sleeps all day anyway' is a mantra used by dog owners to justify their actions (or lack of them), like the infamous 'he only wants to play!'.
There's a good tv programme on 'home alone' dogs here which is well worth a watch even if you already have a dog, especially if you think it's okay while you're out: www.channel4.com/programmes/dogs-their-secret-lives/on-demand/