Price is a major reason why people would choose a desktop chip over a mobile one. It is cheaper, and you get much more performance out of it. Usually, I use my laptop while plugged into the wall, and I very minimally use it on the go.
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The fastest Pentium 4 designed for notebooks, Intel's 2.2GHz Pentium 4-M, lists for $562. The desktop version of the same chip costs $193, while Intel's 2.53GHz Pentium 4 lists for $243 and its 2.8GHz for $401.