@LW_50:
I get what you're saying, but this isn't a case of Stall Warning being inhibited, it's a case of the data being unusable. You can override the SW, but if the failure is of a nature whereby it comes straight back on, it could cause problems.
@HN39:
Retaining the last valid value can cause the same issue - sure, in a scenario like AF447 the values can come back, but in other failure scenarios (damaged vane or cable) SW could be stuck on and cause problems, because the value will never be valid again.