Actually, it's fully possible to use the Steem platform without buying any Steem tokens

Not true, even though the Steem you get for free when you open an account is gifted, it still costs, and is still integral to the working of the platform.

If you power down all of your Steem, your account is essentially frozen, you'll be able to log in, and browse your stuff, but no voting, posting, or commenting. The Steem gives you the bandwidth to operate.

Also If you want to get an account immediately, you pay $10 for the privilege.

Which one of those do you think is the most important one? (or do you see other reasons that I don't see?)

The reasons you list are true, and I don't think it's about what's more important or not, the fact is there are a myriad of ways to leverage your Steem that are becoming apparent everyday.

For instance, you have the bank of @neoxian, he has decided to use his Steem to leverage a financial service.

Then you have Utopian, who use their Steem to fund open source projects. Then within Utopian you can create a funding bot to support your project's marketing efforts.

So Steem, regardless of what you think of Steemit, has a use that extends beyond a Keynesian 'circle jerk'. The fact remains, that Steem is useful beyond being a coin you hold and/or trade.

Cg