The argument is that for the tax system to work, collection should live in a silo: drug dealers should pay tax without fear of prosecution (except for tax evasion), etc. This is the same reason TSA (if its mission were to keep people safe) should not look for anything but threats: we don’t want drug smugglers to invest in concealment technology when they fly.

The downside of the IRS using login.gov is that it theoretically could make interagency data sharing easier. In practice, though, it would probably makes things more secure.