OK465, you might reread your FCOM, the NG FPV is not completely inertial, it does receive baro input and is dependent on the ADC. Simulators might display it differently though as the FPV is not a critical system for operation its simulation often is quite sketchy.
Text from the manual regarding the FPV:
The FPV shows the Flight Path Angle (FPA) above or below the horizon line and drift angle left or right of the pitch scale's center. The FPA uses inertial and barometric altitude inputs. The vertical FPA is unreliable with unreliable primary altitude displays.
If one is absolutely sure that only the speed indication has failed but the barometric input is still correct he can use the FPV in those situations, if only the slightest doubt is there that barometric input might be impaired as well, it is better not to use it.
Aside from that it is of course a nice tool and can be of use in real world flying as well, even if of dubious note in non normal situations.