There are many reasons to overclock, even when the overclock doesn't exceed the single core boost speed. For example, in some cases you might be able to easily do an all core 4.2Ghz OC when the all core boost stock configuration only allows something like 3.9Ghz.
The base clock speed is almost never what the CPU actually runs at under a load, so that's irrelevant. Base clock speed, shouldn't even actually exist because it's rare that the CPU is not running either slower or faster than the advertised base clock if everything is configured correctly.
ANY gains you can get for free or for a minimal investment, are usually welcome. If base clock speeds are good for you, that's great, but they are not for most people with gaming or...