The vulnerability lies within the stage fright itself, within Android. Yes, your Textra app might be patched, but the vulnerability affects any app that uses the media pre-load feature.
Oh, and thats ONE known vulnerability, where Google dragged its feet to even update the Nexus lineup (and the OEMs only gave lip service). Who knows what else is out there.
However, it's your choice. If you don't mind micro managing patches here and there, or dwelve in custom ROMs to get the best "support," by all means. I don't have time for that anymore, and after going through numerous Android devices, I have enough. Do I like iPhone 100%? NO! But the way Google and the OEMs behave forced me to reconsider going back to iOS.
Big Eye Roll...geez.