So, the Facebook phone rumors are back. Unnamed sources say the social network is working with HTC on a customized handset that would launch in or after the third quarter of 2012.

Let’s say right up front that is a pretty damn flimsy rumor – the anonymous source is cited in a story from a Taiwan tech blog, DigiTimes, that then turns the singular into a plural in its headline (“HTC, Facebook jointly developing smartphone, say sources”).

Then TechCrunch took that report and turned it into a rumor of something called a Facebook Phone, which could be something quite different than what the DigiTimes story was suggesting: “jointly developing a customized smartphone in cooperation with Facebook.”

Of course, assuming that rumor has even a vague basis in the first place, that could mean a range of things, from “working together to make sure Facebook can be accessed from a special button on the phone” to “making a Nexus One-type branded handset”. And here are four good reasons why the rumor probably isn’t true.

But, for the sake of argument and because, face it, we all love to speculate, let’s entertain the idea that Facebook does have a phone in the works. Would that be a good move?

There could be several justifications for doing so – although I’d argue they’re pretty weak.

For a start, Facebook’s mobile experience isn’t very good. It’s the Web experience ported to mobile. For instance, just along the top toolbar on Facebook’s iPhone app there are four clickable action buttons (Options list, Friends, Messages, Notifications, Sort) that require either very small fingers or pin-point accuracy. Immediately below that, there are three more buttons (Status, Photo, Check In), with each demanding equal finger-pointing accuracy, which is fine for the young and the sober but sucks for the rest of us.

Of course, it’s not just a UI problem – Facebook tries to shoehorn so much data and so many extraneous features into its iPhone app that the whole experience just becomes cluttered and unpleasant. I don’t need to access my various pokes, third-party apps, groups, and lists – most of which I made once and forgot about anyway – in what should really be a stripped-back mobile environment.

This mobile UI weakness helps explain why Mark Zuckerberg was so eager to acquire Instagram, and it might even justify mobile-first Path’s $40 million valuation round. Andat a stretch, it might explain why it would want to build a phone that has a native Facebook app. Perhaps the companies have ways and means to make a built-in Facebook experience amazing. But then, Facebook would still be facing the same problems: too many data hooks in too-limited screen real estate.

The source in the DigiTimes issue speculates that Facebook might be looking to diversify its revenue streams in order to make more money post-IPO, but that seems unlikely. If Facebook has ever had a hardware element in its business model, it has done a good job of distracting our attention. No, Facebook’s model is instead dependent on collecting as much data about us as possible, keeping it locked inside the garden, along with all our networks, and then using that incredible Web or personal information to let brands zap us directly in the brains with their micro-targeted messages, which fool us into believing that Diet Coke is the nectar of the gods.

Let’s think about this at a simpler level. Even if the argument of diversifying revenues made sense or Facebook had the know how to build a great mobile experience: What would be the point of a Facebook phone? Is there anything a Facebook phone could do that no other phone could? Or even something a Facebook phone could do that most other phones can’t? It’s hard to think of a single reason it needs to exist.

At best, it’s not much better than a Ferrari HP laptop or an old-school Pepsi telephone, unless having all your Facebook contacts in your native address book means that much to you. And to me, having all my Facebook “friends” in my address book would be a net negative, because I want neither the visual clutter nor the in-pocket reminder that I somehow friended that dickwad from high school who flushed my head down the toilet on my birthday (New Zealand schools: rough).

So, basic message: branded phones are for suckers. And even if this probably-not-even-real HTC handset didn’t come with a blue skin and a giant thumb icon emblazoned on the back, it would still be just that, a branded phone. Sometimes branding is more than just the stickering.

That said, a Facebook phone might make just as much sense as a “Google Phone,” which is essentially what the Nexus One was. Of course, Google soon saw fit to focus on the operating system instead of the hardware. Now there’s an idea for Facebook… (But, seriously, please, no.)

Update: A commenter has suggested that Facebook might see a phone platform as an opportunity to segment its mobile experience across different sub-apps. Actually, I could kind of see this working. Maybe Facebook envisages a platform on which it has an app for messaging, an app for contacts, an app for photos (hello, Instagram), and an app for the news feed. Maybe Zuckerberg’s vision don’t entail a centralized presence at all.