I don't understand why you'd even want a contact form over an email link. People have email clients configured and they know how to use them. Their email clients also provide much better editing features for a message than what a standard browser-textarea would provide. Things like rich text (ugh) or auto-save or save as draft and whatnot. Mail clients also usually archive sent messages so a sender automatically has something to reference in the future.

Furthermore, it allows a customer to add the recipient email address to their address book if the need for further communication arises.

On the receiving end, SPAM filtering for email has gotten so good that having a public email address posted somewhere is a non-issue, whereas having to deal with bots submitting contact form is still an issue in need to be solved on a per-site basis, so just having an email link will solve the spam issue for the recipient of the form.

So - why would you still use a contact form? I mean - if it's for a plain contact form - the moment you want to ask for structured data or file attachments, it might start to make sense, but even then - who doesn't hate these forms where you have to click a radio button for selecting the purpose of a message when just typing a subject would do too - especially when the purpose you want isn't listed.

Personally, I always prefer a plain email link over a contact form - both as a sender and as a recipient.