I'm not saying its designed for 30 minutes of use, I'm saying thats the target user.
I spent a few years as the tech support for salesdroids. Each manufacturer has different target markets and though they each call them different names they are all similar. You have the basic home user who works all day then powers their pc on for a little bit to check heir e-mail and facebook. The lower end are targeted at those users and are not designed or tested to be run 24x7. The Latitude line is targeted at the business user who runs their laptop many hours and needs the better thermal cooling.
I never did agree with the big makers choice to use the lower end for the SOHO market be it an inspiron, presario, pavilion, etc. The small office is larger impacted by a desktop.laptop being down than the enterprise market yet those lower end systems are all sold in the office supply stores and those users demanded onsite service and such when their only payroll machine is down. I always tried to steer my SOHO customers toward the latitude/Aramda/Evo enterprise lines for the onsite warranties so that if they did have a problem there was no arguing over me coming to them or them bringing it to me to service.
Anyway the point is those lower end are not thermally tested as much as the business models as the market for them is the user who doesn't really need a laptop (most spend their lives on a desk and never get carried around). You'll find the hinges on those laptops don't last as well as the business models either.
As far as cost, you'll find the business grade models are about the same price because they usually don't have as good as sound/video or all the extra software bundled with them.
Unless your a gamer I always suggest the business models are they are designed to run longer. its similar t buying the right tool for the job, for example an Accura/Lexus/etc can cost as much as a simple box truck but the box truck is designed to haul cargo rather than have a good stereo and leather seats and the box truck will last longer being used to do work every day/