I don't think forcing people to use it makes much sense. If you make it clear that it is going to be allowed, I don't see why people can't choose to play without it, knowing they'll be at a disadvantage, without needing to make that against the rules.
I don't think that "that's the way things are in the rulebook" is a very convincing argument here, where we're discussing what the rules should be. By definition, we're discussing what SHOULD be in the rulebook, and in such discussions, what happens to be there at the moment has very little pull.
In general, it's not like we're going to come to some kind of consensus here. What is important is that people understand the rules they agree to when they enter a tournament, which will help them make informed decisions about which tournaments to enter. And so long as you have to play by the rules you agree to when entering, there shouldn't be any problems.
The only real point I can see for threads like these are: 1) for a particular tournament, you might ask people; however, you're almost certainly going to get disagreement, and whoever is running the thing is just going to have to make a call going in, either way. Which leads us to the more important reason: 2) for all those people who thin that online dominion ought to have some different ruleset than just the dominion rules, they need to codify this different ruleset. I mean, you can do this tournament by tournament, but it's a lot easier (and probably much better) to just have a list of rules that you can say: here, THESE apply. Because for any kind of competition like this, you need a codified set of rules.
Oh, also, in the OP, you neglect to include the very important option of relying on your memory.