MSCs that claim that transfers are not immediate are either being disingenuous or dishonest. Transfers between PayPal accounts are immediate, if the MSC desires them to be so.
The reason why I prefer PayPal is that funds are immediately available. I don't have to worry about ACH delays or checks. The funds can immediately be spent either by PayPal payment or via the PayPal debit card. I also like that PayPal notifies me of immediately of payment receipt by e-mail. I just wished that they charged lower fees for converting currencies.
Sure, ACH payments are fine. But rather than receiving funds into a bank account and then transferring them to my PayPal account, I can receive ACH payments directly into my PayPal account. I just give the MSCs the appropriate routing and account numbers.