I feel like this is just an admission of absolutely not knowing why you buy Wit's End at all.
Why do you buy it? For the DPS, almost certainly. Not only is the attack speed and on-hit actually pretty good, but the MR shred makes it and, more importantly, potentially all your damage except your base attack damage better. Devourer, Sunfire, and Wit's End was an absolutely devastating on-hit jungler build, and Sunfire / Cinderhulk + Wit's End is still pretty solid for a tanky on-hit build.
The protection against magic damage is certainly an important part of the item - there'd really be not much of a way to justify it on anyone who likes it if not for the extra bulk - but it's not why you buy it. It's an on-hit damage item first, an anti-magic item second.