While I think ETCO2 is useful in the non intubated patient (i.e. esophageal tracheal tube, LMA, tracheal tube, etc.) it is worth noting that the jury is still deliberating on this issue.
In the most recent ECC and ILCOR guidelines update they mention that while the use of ETCO2 in the non intubated patient is useful, the true measure of what it actually shows us in this patient population is up for debate.
Consistently expecting the same quantitative data readings that we get with intubated patients just has not been proven or fully studied yet.
There's probably some differences to be expected in quantitative data (which let's face it, is of limited utility without a baseline ABG anyway) but what I care more about is the capnograph tracing.