Voice VPN with or without firewall

I see a lot of conflicting info as whether to put a firewall in front of a SBC or not...

I'm aware of the functions required if putting a layer 3 firewall in front of the SBC (STUN, TURN etc..)

But after seeing some sarky remarks on another topic, with no one really explaining why not to do it, I thought I'd ask why...

is it just not worth the hassle for a voice only VPN, that's why people are happy with a SBC only setup, with the SBC also acting as a zone based firewall, or just throwing some access lists to cotrol access...

curious to hear peoples experiences