Hi ! (I'm the developer of @share2steem)
Thanks for your input.

The goal here is to bring people to Steem from outside. Someone who does the cross-posting will have to come and get their rewards, so they will be confronted to Steem, so one day or another, they will ask themselves : "How come I'm getting money from this ? When I do it for free on Instagram or Twitter ?" and that's where the trap closes on them. We also hope that some of the users will begin to post Steem-exclusive content and change their habits little by little. For now, the problem is that Steem doesn't attract a lot of people, we think we need to broaden the spectrum. It's like everything, a lot of people will use it once and stop, some will use it and be deceived by the rewards and stop, some will become fluent users and get interested by Steem and become active.
Our strength is the numbers. Steem doesn't have the power to attract a lot of users, with traditional social networks, we're talking billions of potential users.

We're also planning on educating users about Steem when we will begin to create accounts to onboard users directly from external social networks.

So it brings traffic and attention to Steem, and if a portion of them gets interested in it, maybe buy some STEEM to power up

And for the 2-way cross-posting, it's also a medium term goal, to have a complete service. And that way Steem promotiom could accelerate. We could also offer incentives to users who promote Steem externally or things like that. With enough users, we could have the power to put a tag on Trending on Twitter or Instagram, imagine the impact...