I'll be using Plastiq. I only plan to use it toward earning large points bonuses in the months right after card signup, to get to the required amount charged. So I might pay 2.5% in fees, but the bonus I'm earning is worth about $1500 in travel spending (Chase Sapphire Reserve card with the in-branch application, 100,000 Chase Ultimate Rewards points).
If I use Plastiq as a boost to help get to $4000 in three months by charging, say, $2k worth of bills I'd normally pay without incurring fees (mortgage, utilities, others), that's 2.5% of $2000 ($50 extra in fees) to help earn $1500 in rewards. Not a bad deal.
I am happy to have a way to keep my friend out of it. He wouldn't be at real risk of self-dealing (or getting in trouble from self-dealing) because he wouldn't be gaining anything from it. But still, I wasn't all that comfortable with the idea. With Plastiq, I'll pay the same amount of fees that I would have been with my friend, but I'm paying legitimate expenses.