When you receive an email, or a letter in the mail, and you look at it, if the font is too small, or too squished you can not read it, then what are the chances that you are going to throw it away? I would say 95%.
As a printer, when I am given the finished artwork to be printed, I do not always look at it because at that point you have already worked on the final piece with your artist and the two of you have approved it to go to final print. The only thing I check for when I am given artwork that way is that it is in the correct format, and there are crops and bleeds, so I can confirm nothing of importance will be lost in the printing or trimming of the final product.
However when my artist is creating the artwork, I work with my customer, and my artist to ensure that all the pieces come together. It is important for an artist to be aware of the fonts that they are using, and to make sure that they are not too small, or too hard to read, otherwise all of their hard work will be thrown into the garbage as soon as it is received by the end user.
My one piece of advice to people today when they are having a print piece designed, is to know who your target market is, so that when you think the art has been completed print it out and look at it on a piece of paper before you print the whole project and make sure that it is legible, and that it appeals to your target market.
What do you do with a piece of illegible marketing material when you receive it?