The description of $it says it CAN be used in a number of cases (see section 5.1.11.4 in the URL conventions document) but it isn't specific about when $it MUST be used. We should clarify when $it is required. For example, my understanding is that both of these queries are equivalent (in which case the $it is not required):

http://host/service/Customers?$filter=Orders/any(d:d/Quantity ge Age)
vs.
http://host/service/Customers?$filter=Orders/any(d:d/Quantity ge $it/Age)

However, my understanding is that in the case below the $it is required:

http://host/service/Customers?$expand=Orders($filter=$it/Address/City eq ShipTo/City)