Oluf
In my (Trent powered) 777s the norm is for engine anti-ice to be in "Auto" (I assume other operators do it similarly?): when the ice detection system decrees, the A/I valves are commanded open or closed.
MEL aside, "on" is normally only used on the ground, when conditions dictate.
Even if, in an idle descent, there is not enough bleed air from the LP bleed, there is always the HP bleed- that is, pretty much, why it is there. Even if HP bleed were not enough, when the A/I valves are commanded open the EECs command "approach idle" and a good bit more bleed air is available. So no problem with lack of bleed air for engine A/I in a descent, I think.
Having said that, there is an FCOM caution to avoid prolonged operation in moderate or severe icing conditions.