Hi KristianNorway,
Why select isolation valve to "close" at all?
It's to ensure the power available on the surviving engine, after a severe failure of the other engine with damage to the pneumatic ducting causing a bleed leak.
Look at the performance penalty of packs on, engine anti ice on, wing anti ice on. If you take off with the X Bleed open, suffer an engine failure and there's a rupture in the ducting - then the surviving engine will be pouring bleed air through the rupture. Big penalty.