I have the L3, but had more than 4 outlets, and wanted to control my kalk dosing with pH monitoring.
If you want timers, the L1 is a great starting point. If you expand your system and find you need more outlets, or more control, or just want more "stuff" it's expandable to a total of 4 modules.
I've been running my Reefkeeper Lite since May of last year and can't imaging running a reef tank without it now.