The harm is that you cannot identify who is paying you (everyone can see your incoming transactions and can try to claim them as their own) and that everyone else can identify activity to you.
Better: generate a big file of addresses (try the "bulk wallet" utility at
http://www.bitaddress.org which can generate thousands of addresses in your web browser in CSV format) and dispense one to each customer. The private key (the field with a number starting with "5") is the code you need to respend the incoming payments and should be kept secure, preferably offline, definitely off the web server if you can help it, because this is the part any hackers will be after.