The main trouble is lactobacillus - which is pretty much present on any grain you have. Leaving the mash sit for an extended period will open you up to the possibility of having a tart/sour taste in your beer -even if you boil it the next day. Search up on how a "no boil" berliner weisse is made if you want to get an idea of what's going on. If the temperature of your mash stays high enough the entire time, you can keep the lactobacillus at bay for a while.
The second trouble you might have is a very fermentable wort -leading to a dry beer with a low FG. The mash enzymes will convert everything they have time to chew on - and you're giving them lots of extra time. Most people do a mash out at a higher temperature that will denature enzymes - in order to keep some "body" in the beer with a more dextrinous wort. Since you're doing a partial mash, maybe that isn't much of an issue, because the majority of your wort will be comprised of extract.
That said, some people do what you're talking about & seem to be happy with it. Give it a try if you want - it's your beer.
