For marketing, which has been positive for a lot of artists. It's definitely allowed people to get their works out there, but let's not act like it's for serious photography.
> but let's not act like it's for serious photography.
I (strongly) disagree. There is some incredible work on Instagram from (many) thousands of 'amateur' and professional photographers alike. "Serious photography" isn't just restricted to DSLRs/MF/etc.
In many ways, the constraints of mobile photography (lower res, fixed focal length, small viewport when consuming) has benefits, just like constraints elsewhere in photography.
(I follow many photographers who primarily or solely publish to Instagram; many who aren't marketing their paid services.)
They are using Instagram as an easy publishing platform. It's assured the big ones find it very lucrative. The biggest issue with Instagram is all of the bullshit accounts with millions of users, that are just posting the work of other artists with a "credit" if they are lucky.
I would even argue that Instagram has in some ways redefined professional photography (by enforcing different constraints than a professional photographer is used to).
Aggregator accounts are one of the best ways to discover high quality users.