IIRC, the ignore system on OldGuild showed an ignored post as something like

[b][IgnoredUser posted - [color=aba000]<Show Post>[/color]][/b]

thus accounting for instances where an ignored user makes a post where you have to acknowledge them.

This is mostly a feature for the OOC sections, allowing you to filter users that rustle your jimmies. A whitelist system really wouldn't work there. There's also the application of filtering threads in the Interest Check section; not having your options clouded by several low-quality threads drawn up by a single person. Not going to get into specifics for obvious reasons, but that example has been something that I observed on the OldGuild quite a few times.