I'm not a marketing expert either, and I have no idea what the real difference between a catchphrase and a slogan are. Maybe it's worth researching before I do a follow up post.

You make some great points about what the product is or may be, and you're absolutely right that it will change and develop over time.

Different users will have different perspectives.

I think you should try to write some slogans/descriptions that encapsulate what Steemit is for you. For me, I see it as a platform that can empower individuals by connecting them to an audience and rewarding them for doing what they are interested in. Of course there are other applications and way Steem can be used, but I am beginning to feel that Steemit is a platform like I just described.

If you come up with some slogan/description ideas, share them here. (I know you said you don't think they are necessary, but in case you change your mind.)