Batch sparging in a rectangular cooler you may loose almost a couple litres of 1.025ish wort.
Tipping the tun also disturbs the grain bed.
Tipping the tun isn't consistant I reckon, there's a million other variables in my brew day and the more constants I can have the better, I like to know if I put x amount of water in that I will get y amount of wort out.