NEW YORK ( TheStreet) -- Amazingly, nobody has yet pointed out a severe usage limitation on Apple's ( AAPL) iPad that surely will cause grief with the very first reviewers. I'll get right to the bottom line: Just like the iPod Touch, the iPhone and most or even all other smartphones, the iPad lacks multiple user profile logins, including any "Guest" login. Think of the iPad as one big iPod Touch or iPhone. Once you've entered the password, you're in. And I mean in! You have complete access to all emails, instant messages, the address book and calendar. Contrast this with a laptop: On a PC, you may have, say, four different user logins (father, mother, son and daughter) and one generic "Guest" login. This means you can't see others' emails, instant messages, address books, calendars and any other documents created. Privacy is protected. So why is the lack of multiple user login or a guest account such a critical flaw on the iPad, when the world of smartphones doesn't seem to have crumbled in the wake of a similar deficiency? The answer should be obvious, but I will spell it out anyway: Unlike an in-pocket smartphone, the iPad is almost naturally a somewhat communal device. Where will you find the iPad most times? On the table, not in the pocket. What is its main purpose? To surf the Web, among many other things, of course. There is almost an expectation that anyone should be able to pick it up and use it. Just imagine the regular family of four: Residing on the dining table, kitchen counter or coffee table, the kids in the family will be jumping for the iPad at every moment. Will you tell them, "No?" Can you imagine their loud screams? For the iPad to have meaningful utility to a productive adult, it needs to be synchronized with your personalized data using iTunes and Apple's MobileMe. But wait. By containing all this personalized data, including your emails, address book and instant messages, just for starters, the kids will be only seconds away from destroying your most valuable information -- often work-related -- on the iPad. You can see the headlines right now: "Kid Gets Password to iPad ...." Fill in the humiliating blank.
The fact of the matter is that the iPad is a lot more like a laptop than a smartphone in terms of how you need to protect your information. You wouldn't let your kids use your laptop under your personal login, with access to your emails, address book, documents, and instant messages. This will force parents and others to not sync their personal information -- through iTunes and MobileMe -- with the iPad, at which point the iPad has immediately lost a material portion of its intended utility. It should be very easy for Apple to fix this critical flaw. In fact, Apple needs to do the same for the iPhone and the iPod Touch as well. You want to be able to lend your iPhone and iPod Touch to someone -- no matter how temporarily -- knowing that all this person can do is to surf the Web, depending on how limited you have set the permissions. The same goes for all the other smartphone systems - Blackberry from Research in Motion ( RIMM), Google's ( GOOG) Android, Palm ( PALM), Nokia ( NOK) and so forth. Until such time that Apple fixes this critical flaw, however, this gives rise to what I predict will be the biggest complaint about the device, with the possible exception of the much-feared lack of ubiquitous multitasking. As it stands, no user in his or her right mind will dare to synch the iPad with iTunes and MobileMe. And there goes much of the utility of the device. And for those who dare, they have just signed their own personal information death warrant courtesy of their kids. Someone told me that the solution will be for that family of four to simply buy four iPads. That may even be Apple's secret plan to beat all of its revenue estimates! Please. If there was a more obvious way to upset your new iPad owners, I can't think of one. You have just spent $500 to $830 on a device that's fundamentally less capable than a $300 laptop and immediately you're told that you need to buy four of them in order to protect your personal information. Please! In the movie "Armageddon, one astronomer declares "We have 18 days!" until an asteroid hits Earth. Indeed, Apple, you have a April 3 deadline to fix this. You can do it. I hope. Or else the reviews won't read so well. At the time of publication, Wahlman was long Apple, Palm, Research In Motion and Google.