Normal DIMM slots hold the DIMM quite firmly even without the locking tabs requires some seriously rough case handling to make them fall off so their apparent primary purpose is not actually that important - at least one of my ASUS boards uses DIMM slots with only one "locking" tab.
Those tabs serve a secondary purpose when REMOVING DIMMs: many models I have seen have a nub at the bottom of the slot that pries the DIMM out of the slot when you open the tabs - those are the same nubs that make the tabs close when the DIMM is inserted. This is to prevent people from having to pull on the DIMM slot when removing DIMMs, which could break solder joints or even rip the slot off the motherboard.