The rules for faster memory also mostly apply to more bandwidth (read: more memory channels): the usefulness of it will depend on the system and purpose of the system. Based on the benchmarks we performed, we'd say that the benefits you get from more memory channels increase when the speed of your CPU is lower. Without a doubt you will benefit the most from dual channel memory, when you are limited to the integrated GPU of the processor. Games run noticeably smoother with dual channel enabled, compared too single channel. Therefore it is all the more important to pay good attention to this when purchasing a laptop.
The step up from dual to quad channel barely leads to faster performance, at least in our tests. The only test with a significant was the WinRAR test, and it should be noted that the performance improvement wasn't that huge. This is no reason to ignore potential extra performance by not equipping an X99 board with four memory modules (since there tends to be room for eight modules), however this does indicate that you don't lose much, if any at all, when only using two slots. And this is good to know when you opt for a board with only four slots or less, such as the aforementioned ASRock board.