Recently, I was asked by a customer about the benefits of SIP Trunking.  Before I respond to this, it's important to understand that SIP Trunking will put a dent in the concept of a classic VoIP gateway.  The SIP Trunk term refers to the connection to the enterprise - it's a SIP-based Trunk coming in as opposed to a PSTN (T1 or E1) trunk.  The border element concept will remain, but converting from TDM to IP won't be necessary.  It's more about IP to IP - an IP to IP gateway if you will, with mediation / security services. This is what I've been referring to as a border element for quite a few years.  In other words, the classic VoIP gateway evolves to be a different kind of border element.  If you want to read more, the TMC website has a Global Online Community dedicated to SIP Trunking.

So with that in mind, the benefits of SIP Trunking really relate to cost.  First of all, assuming the enterprise already has internet access, then you already have the physical connection and you are already paying for this.  There just has to be a software based provision for the "voice" trunk.  In the end, you are really just utilizing the internet bandwidth more efficiently for both voice and data services.  That's what it comes down to.  

Additionally, it's probably also easier as an IT manager to upgrade the number of users you support as your business grows.  Not as much hardware required.  And ultimately, this will enable enhanced unified communications since all the traffic is coming over the same line.