I use L3 as well.
Your thumb is always on the left stick because steering, so if your thumb never leaves that stick and pressing the stick in can serve an important function, it makes total sense to me. I sacrificed my ability to switch between low/high beam because that was useless to me.
It enables you to press it while still having full control over all other buttons and functions, so as to drive consistently and cleanly while engaging DRS (based on L2/R2 for throttle/brake)
Of course everyone is different, so each to their own, but that's my 2 cents.