I was an LEO for 20 years and only carried (under the seat) a back up 9MM semi-auto when we had a hold up man robbing banks using an Uzi. At the time, all were were allowed to carry were .38 revolvers and I figured speed loaders were slower than another gun.
Is a back up a good idea for a LEO? Probably, but I could not afford one at the time.
I am now retired and do not see the need to carry a backup gun, although I do carry an extra mag when carrying my 7 shot 9MM.