I dont see the problem with having a static ip, they are obviously needed if you have an email server, or web server or something but you could get around thus with dynamic dns, anyway i would go for it if you have a NAT firewall/router running all ports in stealth you will be pretty much invisible anyway, you could take extra measures however such as making the user for your ftp server some randomly generated alpha-numeric string, and you could change the password and username frequently. but anyway in answer to your question, if you take proper security measures i cannot see any downside to having one