I just recently had a client whose entire house was painted with a flat finish paint. She was complaining about how dirty her walls were and how it didn't wash off when she tried to clean with a damp cloth. Unfortunately, she was unaware that her walls were painted with flat finish paint until I explained to her the reason why her walls looked so dirty.
Flat paint CANNOT be washed. It is very porous and soaks in water or any other scuffs or staining. The only way to really fix it is to touch it up with more paint. And even this is not always the best idea, because the color may not come out the same as when it was first painted, so you may have a slight variation in color.
The good thing about flat paint is that it is non-reflective, so it tends to camouflage any surface imperfections, such as dents, cracks, bumps, or any other rough areas. It is the best choice for ceilings, because of its low reflectivity.
I get mad over this situation, because a painting contractor can easily choose to use a glossier paint sheen, such as eggshell, satin, or pearl. These are easily washable with water and do not need to be touched up for many years to come. There is also a washable matte finish paint that Benjamin Moore has recently launched that has the look of flat finish paint but it can be easily cleaned.
Please don't make the mistake of not being informed about the different paint sheens that are available to you. Some painting contractors are only doing this so that you will hire them to come back and paint your house all over again within a year - especially when you start noticing how dirty your walls are getting.
The key to know, is that the higher the sheen - the better the durability