I use 2x8s to raise my beds. Our soil is incredibly rocky (they call the island the Rock for a reason), so to get some decent space to grow things like turnip, potato and carrots, I have not choice but to raise the beds. The surface area is not much more. There are no holes or water escaping through the lumber. No voles or moles either.
I guess that's the whole point. Every location really lends itself to a certain type of gardening. Loosening the soil six to eight inches down here is a major chore...so we go up. Water retention is really a secondary concern...especially since it rarely gets hot enough for prolonged evaporation to be a concern (average 21 in the summer).
..
Zone 5a in Canada's Cool Rocky Far East.