IAX, and more specifically, IAX2, which is the version of IAX currently is in use, does not need any type of inspection because audio and signalling are unified, unlike SIP. To allow IAX traffic through the ASA you only need to allow traffic to UDP port 4569 in your inbound ACL (plus the static to forward this traffic to your PBX).
Regarding the advantage of passing IAX2 traffic instead of SIP traffic, well, that depends on what your PBX uses. If your PBX only uses SIP then there is no point in letting IAX2 go through. If your PBX can use both SIP and IAX2 to talk to other PBXs/devices the IAX2 seems like a better option.
Take a look at this good comparison between the two written by one of the authors of the IAX protocol:
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...