没有验证码工具的联系表格的利弊

Question

I am providing a simple php contact form on my webserver. The form include fields for name, email, message and one hidden 'do-not-enter-anything'. If the 'do-not-enter-anything' field has some value after user input, the server will not send any message. I do not use a captcha tool. Which problems could occur without the use of captcha? I am not afraid of getting randomly spam messages. Is it possible to knock out the server with the contact form and a script (on an easier way)? Are there other disadvantages?