Plus don't the spambots enter their URL's or other bs into any fields they find, whether they can read the text descriptions for that field or not? Even if you code it as a required field, the spambots will enter data into it to get the form submitted. That was my experience awhile back when I didn't have any protection implemented but had required fields. So you'd have to code the form to recognize specific formatting of what's entered into the field before they can submit.

When dealing with spambots the degree of validation required is going to be dependent on the popularity of the site and/or the script you are using. A very simple form of validation will be fine if your script is homegrown and the site is not very popular. If your site is very popular or you use a recognizable popular form script, then the incentive for a spammer programmer to find a way past the validation is greater. Even with sleazy spammers it is all about return on investment, they are not going to invest the time to find a way to spam your forms unless there is a good chance for profit in doing so.

In a similar manner you must decide if your choice of validation is worth the effort. As mentioned, any sort of validation is going to have ramifications. The more hoops you make user jump through, the greater the likelyhood of the user abandoning the form. This is especially true when your target audience is not technically sophisticated!