Spanner Turner said:
"Why is it number #2 crossfeed that most operators use to keep the crossfeed manifold pressurised?"
This is simple. the philosophy here is to protect if the manifold breaks. By leaving #2 open, if a fuel leak develops in the manifold, and you discover it, close the manifold off completely. At least you can transfer fuel using the dump valves back into #2 (or #3) so you can balance it back out or save yourself from starving the engine.