Actually, sometimes I'm surprised that people will contact me through whois rather than my contact page on the domain itself. Somtimes they'll do both.
Almost _never_ will I get contacted through the CIRA contact form. Only an experienced domainer would even know that exists, good luck having an end-user ever find that form!
If you're in the business of selling, then I agree, you want to give any potential buyer any and every way possible to contact you. Furthermore, if some lawyer thinks they have a claim on my domain name, I want them to know I'm not hiding and that they can contact me easily, before they start off with a CDRP. Even if they have no chance of winning, I'm forced to either pay a lawyer or put significant amount of my own time defending that kind of stuff, so I prefer not to go there unless I have to. That being said, if you always have a contact link/form, then needing whois being public is less of an issue. If you're _relying_ on whois to be your contact info, then you're making it impossible to contact you for a vast majority of the uninformed public. A contact form is definitely the first priority of you're a seller.
The main downside I see to having your whois public is your competition - other domainers. I've found that there are a few people who will stalk your every move and not having privacy makes it easy for them to track what domains you're buying/selling/etc. Some of those people are probably reading this right now, so you know who you are
I'm not saying I blame them, nor would I say I don't keep track of stuff like that myself. Information and knowledge is power, and it can be used against you too. So consider that. That's more of a function of how paranoid you are though, and how big of a target you are. Most domainers probably shouldn't care too much about that. People with large portfolios who are very active buying & selling, they're the ones being watched closely.
