The implication seems to be that it will be easy to switch to a different dial-out provider, of which there are plenty. (Mostly marketed towards businesses who want to use VoIP for all their telephony.) This is a good thing.
In my interpretation, “No-Carrier” (presumably not to be confused with NO CARRIER) basically means you want to decouple your ability to dial out into the PSTN from your network connection, whether that network is a cellular network, WiFi, or something else entirely.
When Todd mentioned that the one disadvantage of going “over-the-top” is that there are some situations where a native GSM call will still work but VoIP will not, it made me think of an existing service that could provide the best of both worlds; your ability to dial-out can be decoupled from your network connection while retaining the ability to place native GSM phone calls. It’s called SIP2SIM®, from Andrews & Arnold. You get a SIM card from them which lets you make GSM phone calls, but, after travelling through the GSM network, the calls go via the SIP server of your choice instead of directly to the PSTN.
In practice, SIP2SIM is probably not economical for most individuals who don’t want to use it as part of a business phone system. (In addition to the call charges, data is not very cheap on the SIP2SIM SIMs.)