The base clock speed is a guaranteed clock speed for full typical (but not peak) utilization. As long as one is not running a power/thermal virus (i.e., not peak utilization), one should be able to fully use the hardware at the stated clock speed without violating the thermal design power.
When software is not using all the available cores, some additional thermal headroom is available which can be exploited by increasing the clock speed of the active cores (Turbo Boost).
Incidentally, minimum clock speed is not especially useful for evaluating energy efficiency since a significant amount of power consumption is static power (independent of the work done). At a certain point, race to sleep (a.k.a., hurry up and go to sleep) will be more energy efficient than reducing the clock speed because sleep states reduce the static power use.