I would say that the issue, in my opinion, is that given x amount of dollars for expansion, how much money per square metre should be spent? At one extreme, we have the Libeskind approach -- very high visibility architecture, but relatively little actual extra floor space, relative to total cost. On the other would be the 'Four Seasons Centre' approach, with a lot of extra floorspace, but unimpressive from the street.
(I am aware that fundraising is easier with a Libeskind-level proposal -- a warehouse might have cost less, but also less money would probably have been raised)
On the other hand, there is such a thing as having too-impressive architecture. I was in a few Spanish museums that were converted Medieval or Renaissance palaces, which completely overshadowed the collections housed within. More of the visitors were looking at the architecture than were looking at the paintings.
The Libeskind extension increased exhibit space by what, 30 percent? It was, however, about the most expensive possible project, on a per-square-metre basis. I personally think that even if the ROM was only able to raise three-quarters of the amount of money they did, for a less spectacular but more economical design, they could have increased floor space by much more, maybe 50 or 60 percent. Since I am mainly interested in the exhibits themselves, that would have been a good trade to me.
Others may have different priorities, of course.
Bill