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.

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