The stated reason is that quality of service cannot be guaranteed with more than one modem since it's operating from the same line, but as you say a line with a strong signal can support more. Getting them to set up the account, on the other hand, is the hard part... sales won't do it once they realise there's a modem already at the address. Although, you could maybe pick up a modem at Virgin megastores and do it by that method, but to be quite honest I wouldn't recommend it when it's so much easier to just use a router.