Then what's the point of broadcasting a 'subscription' transaction with an nLockTime days or months into the future, if the transaction is just going to get dropped long before the time it is 'due'? Or am I misunderstanding the suggestion that was given for using nLockTime transactions for subscriptions?
None. But you don't have to broadcast it. Sending it to just the recipient is sufficient.
Of course, I could still keep miner's disks pretty full if I broadcast say 1000 nLockTime transactions an hour. Basically if i want to force miners to waste 100 MB of disk space, I just have to transmit 100 MB of nLockTime transactions per day; each day when miners eliminate old cruft, they just get more from me. I get some 'friends' to do the same and now miners are wasting lots of disk space and burning lots of network bandwidth accepting these transactions.
They'll just add rules to drop those transactions. It doesn't matter if miners hold them. So long as there exists some person with a financial interest in those transactions, they'll hold them. And one person with an interest in getting them into the chain is better than 1,000 people with no such interest.