If you are only concerned with 1 gmail address by all means use the gmail app and ignore the generic email app. The gmail app is far more useful and integrated for gmail addresses.
Have you tried attaching a document to an in-process reply in a conversation thread in the Gmail app? Want to bold some text? Want to use Email Encryption or Signing from your mobile device? Want a universal inbox?
What exactly is your definition of "more useful...for Gmail addresses?"
The Gmail app is terrible. The only real benefit of it is that it is designed for the way Gmail organizes email (Stars, Labels, Conversations, etc.). Other than that, it's terrible. It was great back in 2010 or so when most devices had pretty bad stock email clients, but that is no longer the case. Productivity-wise, Gmail has fallen behind those clients and it's about as good as Apple Mail was on iOS 6 - maybe even worse than that...
On OEM devices, like those from Samsung, the Email App is far more powerful, more useful, more capable, and - yes - supports conversation threading (and text formatting, and certificates/signing/encryption, better notification and sync controls, etc.). Gmail exist primarily because Gmail usability is subpar in mail clients that aren't designed specifically for that service, which is why a whole market for Gmail clients sprung up on platforms like iOS before Google released it on that platform.
Gmail has to be used for consumer Gmail accounts because Google does no longer support EAS for those. If you want your email pushed to your phone you have no choice (or you can have it forwarded to an Outlook.com account), and if you want anything resembling decent organization you don't have much of a choice. Gmail through IMAP is a complete PITA especially if you have a lot of Labels or Nested Labels.
Even if I actually used Gmail, I'd still have it tied into the Email app as an IMAP address with manual Sync and only to download the past 3 days or less of email just so that I could have a better client in which to compose emails, or send secure email if that was necessary.
Gmail: Great Viewer - Poor Composer
Email: Great Viewer (though not as aesthetically pleasing on some devices) - Far superior composer to Gmail.