The NAV radio side of the 650's output is only transmitted over ARINC-429, which is what the HS34 has in it. Without that, you won't be able to see any VOR, LOC, or GS information. Though we can still show GPS information via serial input if you don't opt for the HS34, the needle won't auto-scale (because the information to do that isn't in the serial data, only the ARINC-429) and you won't see any GPS glide path information (again, because that information is not on the serial output, only the ARINC-429).