Yes, that makes sense. Thanks for your contributions. I guess that once you are in the situation it is a rather straightforward.
I was thinking of a critical situation, like flying in a A330 (deaereated grvty ceiling is FL200) to an isolated airport or something similar, where the nearest en route alternate is quite far.
I thought that, somehow, delaying the procedure by using the X FEED was equivalent to turning some fuel into unusable fuel.
After this thread I undestand the whole thing better now. As a matter of fact, when there is a fuel imbalance, the A330 has a curious way to transfer fuel between inners via the refueling gallery when the X FEED is inop or closed. It also requires banking to the lighter side. This would permit full use of the fuel and maximum delay in the execution of the gravity feed.
The banking method is part of an IMBALANCE procedure in the 330, while it is part of the GRVTY FEED procedure (OEI) in the 320.
As a last note, the A320 procedure for feeding an engine by gravity by the opposite engine, I don't think it would be advisable on a two engines case, since we could starve the other one.