I could create a ledger with some paper and a pencil, calling the numbers in the ledger "nums", and after someone fulfills some requirement I can write +50 "nums" to their account. Then, for example, they can then have me write -25 "nums" to their account and +25 "nums" to someone else's account. But what direct service do "nums" provide? Why would anyone want to pay a cost (whether it's meeting some arbitrary requirement or offering a good) to acquire any "nums"? The only difference between "nums" and bitcoins (and this difference is actually irrelevant) is one of who or what is trusted. Even assuming you had full trust in me and my ability to keep the ledger accurate and secure, "nums" still provide no direct service to anyone. My ledger service has the potential to provide a direct service, but the "nums" it keeps account of do not provide any direct service, so would my ledger service actually be providing a direct service to people by keeping account of "nums"? I don't think so.
You are right that it all revolves around trust. The fact that you can buy something for dollars (or gold) is because other people trust that whey they in turn need something they'll be able to turn the dollar they received into something else. This is the same case with bitcoin. Dollars don't provide any what you call 'direct service' (if you don't use them as a wall paper that is), or 'use value' (in Marks terms I think), they only value is 'exchange value'.
I still disagree that the ability to pay taxes is not a direct service. I know that I need to pay a certain amount of taxes. So, for me, the dollar is backed: this 1000 dollar bill will be able to pay off 10% of my taxes, so if I get 10 of them they have served a direct service (me not being in jail).
Currently, Bitcoin is not backed this way. However, it can be. That's the extent of my argument.