IPO has a branch mode to provide something between 'full' failover with a LSP and the most basic SLS functionality in a G450.
If you ever worked on CS1k and know it's SMB cousin BCM, there was a very similar survivability setup with those calling them Survivable Remote Gateways - or, SRGs.
With CS1k and SRG, you had to have all Unistim IP phones - no TDM.
With CM and IPO, you have to have all IP phones be SIP, no h323.
With CS1k and SRG, you had to mirror all programming manually across the two systems - to say, build every extension and password twice.
With CM and IPO, you have to do the same, although it's somewhat more streamlined with System Manager, though it has its awful problems.
With CS1k/SRG and CM/IPO, you failover to the call processing of the branch gateway, and since IPO has no PPM, you can't have things like a bridged appearance
In any event, if you have an environment with a lot of changes, IPO with SMGR integration is more suited to day to day administration.
If you had a network of 1000 retail stores or bank branches and reception was 100, wicket 1 or cash 1 was extn 101, etc and everything was always the same, you'd get the same functionality from a telephony standpoint by having an Audiocodes that let SIP phones register to it.