I've said elsewhere on the forum that it's possible that some users or search engines might see paying more than you have to for your hosting infrastructure as a sign of legitimacy. Paid certificates could be in that category. That is, it's showing that you have an infrastructure budget—something that perhaps certain scammers or simply less-established businesses don't have.
On the other hand, this signal is probably relatively weak and easy to fake.
It's interesting that the low barriers to entry for web hosting (which Let's Encrypt has help reduce further!) could be viewed as a problem by some businesses which are used to being able to create indicia of legitimacy and trustworthiness either by spending money or by showing that they have the capacity to be sued (or punished by regulators) if they engage in wrongdoing. Those signs are harder to replicate online.
In the present moment it seems like some people are regretting the ease of online publishing and interaction and wishing that it were easier to create barriers to entry, or identify who was behind a site or account, or punish people for online wrongdoing, or quickly shut down allegedly fraudulent sites, or otherwise have more friction or more "substantiality" in the online world. It's easy to sympathize with some of those concerns, but PKI is probably not going to help solve most of them—if anything is.
Let's Encrypt was mostly made by people who like that it's really easy and cheap for anyone in the world to make a web site with minimal resources and few preliminaries, and have that web site be on par, in some ways, with a professionally developed web site from an established institution. That also has its major upsides, and the Let's Encrypt community is likely to focus on them.