Not to mention whmcs quirks.
None of the hosts I use that pro-rate use WHMCS to send their monthly invoices...
I wish I could, but I'm thinking it'd cause a lot of problems to do it with dedicated servers.
Why? (I'm asking because 3 out of the 4 dedicated server companies and 1 out of the 1 colo company I use have pro-rated billing).
It might depend on who your dedicated server customers are:
1. real businesses who tend to pay by credit card=it'll work;
2. low end customers (the type that gravitate to the likes of CC/CVPS/GVH offers), pre-teen summerhosts, etc. = tend to pay by PayPal, BitCoin (or their mother's credit card), forget about even trying to explain pro-rated to this group.
Nobody's mentioned it in this thread yet, but one of the benefits of pro-rated billing is you only pay one (or two if you bill bi-weekly) nightly credit card batch fees per month instead of 30.