One strategy might be to signup as many free 10 SP accounts
The free offer is "$10 worth of SP" (i.e. 3 SP at present) not 10 SP. The 10 SP may have been the offer at one time but not any more.
Ah I see. So the point remains that users need to want to stay on the site, else they will have many abandoned 3 SP accounts.
No doubt. Of course nothing prevents those users from coming back later to claim the accounts if the value goes up. There is no maintenance cost to idle accounts that don't use bandwidth.