why etc
It reduces the pressure surge on rotation (change of aircraft attitude , ground effect etc).
As for affecting pack life, I'd say it would be negligile.Air bearings etc.There is a risk of loading up valves with the surge of pressure at climb power though.
Had a 320 pilot cycle pack s/w's once at altitude and blew every presure blowout panel out of its mounts.Whoops!
Not using packs during t/o imho is just a real "tight anus" way of saving a few pence on fuel.