I think it may also intended to convey that the author of the sentence may not agree with the statement. Maybe more than 'not a few' consider it, as in, 'many', but the author considers those 'many' to be incorrect in their usage so therefore they highlight this disagreement with the use of 'not a few'.
You could also have the complete opposite interpretation. I.e., they agree with the 'not a few' and are trying to use hyperbole. I.e., "not a few people consider wife-beating to be wrong."
It really depends on the context of use which is really hard to determine with what is provided.