My one question is why it doesn't always come up?
I think it is because of different sims and the ECAM standard they are imitating.
Here's a picture of an ECAM line during EFTO taken in one of our sims.
Notice how that ECAM standard then invited you to open the X Bleed Valve (ON).
villas,
It has nothing to do with anti ice
You may believe the Status invitation to open the X Bleed Valve is for Pack operation only, but I believe it is also there to enable symmetrical wing anti-icing.
If wing anti-ice ON:
PACK (affected side) OFF
One pack must be closed when wing anti-ice is in use due to precooler performance.
If ENG FIRE pb not pushed:
X BLEED (if ENG FIRE pb not pushed) OPEN
X BLEED selector must be opened to have symmetrical wing anti-icing.
Single pack operation (or using the APU for the packs) presents no problem.
Wing Anti-icing is only available from an engine bleed and with X-Bleed Valve open. If you don't have wing anti-icing and you are in icing conditions, then there is a Landing Distance performance penalty (due increase in VAPP).
Single Pack Operation doesn't affect Landing Distance.