More teeth on the cogs is measurably more efficient. The did all the analysis in
Bicycling Science but I don't have my copy handy. But go pedal around in your 11T cog sometime and you'll feel it. Other reasons are to prolong drive-train life (more teeth and links to share the load) and more teeth wrapping around helps keep your chain on. Downside is a bit of weight and less chainring clearance for your trials moves.
The good folks at Surly bikes have
a pretty good take on the whole issue too I highly recommend it.
I run 34x19 as my normal gear for a 29er. Also have a 21T cog for loaded bikepacking although so far I have been too lazy to change it out for any trips.