really? i've always counted weeks, since every week is the same lenght (and paid fortnights, since every fortnight has the same lenght). i get paid by the month now and it's super weird to me, as if i'm worth more in february than in march.
From a company standpoint, not having to accrue the liability on the ledger simplifies the accounting auditability & ability to close your books monthly.
On the other hand, I understand people who are on tighter budgets who would rather have their revenue better aligned with their expenses rather than having to balance them on their own.