I had this same question since seeing the post about the Fossify phone app. For phone, calendar, contacts, things like those, i dont see what value the official google apps have (other than syncing to your account, but i can manage that myself). For Messages from Google, tho, there is something they provide in the RCS, if only because they block others from implementing.

So, using Fossify Message, for example, sacrifices something of some actual value here...