Notification of this post popped up while finishing the intro to my #Roadmap2018 draft which will focus around the fundamental economic 'backwardness' of STEEM becoming sustainable and thus scalable. Thus I'd add my support for this relatively minor 'feature' in its contribution to resolving the larger problem of 'Revenue generation' for the currency to function in rewarding 'valued content'... 

which I guess I should briefly summarize as the exclusive function of leveraging of USER-ATTENTION in satisfying [their] objectives/interests, which is addressed:
1. FIRST by AWARENESS (the design by which posts/topics/data may eventually reach an individual that values such particular items) &
2. THEN the ability to TAKE ACTION 
A-KA "MARKETING".... as we must speak in terms of business, 'ACTION' would mean buying a product, but analogizing this closer to STEEM's content based function-context it could mean Voting, commenting, tagging, reporting; simply participating in some way that generally adds value to the system/network

So to avoid over-complicating this hat-tip of a comment for the particular idea of 'Pay-Per-Comment' or in reality 'written Advertising slots'..., I'll end by re-iterating the  VALUE-GENERATION spawned by your idea of 'Pay-Per-Comment' as an OPTION granted to creators is both in:
1. Increasing the NET REWARD accumulated by QUALITY CONTENT by further leveraging that content's ACCUMULATED ATTENTION & particularly the very specific/targeted AUDIENCE around any given post
2. Offering an additional method (if taxed by the platform) to RAISE GENERAL FUNDS (aka Revenue) that can serve to generally support the mechanics of this content-distribution & reward system where everyone wants to profit but very few are interested OR ABLE(/Incentivised) TO SPEND (because there's too few scenarios for spending to result in a specific value/exchange aka purchase of product/service/info)

PS- Right as I was about to end this rant I thought of the added implementation of 'turning on paid-commenting' being that the comments are then somehow more predominately displayed, perhaps via a customization mix of AD-like display configurations throughout the TOP-MIDDLE-BOTTOM of the actual post via HORIZONTAL banners, Verticles, embedded box ad formats, etc where the those interesting in paying to comment can maybe even have greatly option in HOW their text is displayed, like teaser/preview mode for longer text OR enabled images when default is then disabling embedded images in comments. There's a ton of opportunity to leverage here for greater demonetization of 'VALUE' created on STEEM by 'POSTS' and the users that actually CONSUME them.