We chose to number Customers because we were originally imported them from a legacy system and we wanted to easily identify the source and for customers to use the same account number. We know that Customers starting with 1 originated in Legacy system 1, Customer starting with 2 originated in Legacy system 2, and Customers starting with 7 originated in Netsuite.
One annoyance is that on reports and saved searches, the account number and name are usually concatenated, even though they are separate data fiels on the customer record. I'm sure there's a way to avoid this but we haven't taken the time to look into.
We don't number vendors. AP wanted to manually create the vendor ID as an abbreviation of the vendor name.