Inventory module is a big package that has been designed some time ago and it has invoices built-in. Recently we've designed standalone Invoice module that will be used by other modules - by Warehouse also. This invoice module will allow you to simply create invoice with any items you wish. Warehouse will create invoices using this module in the future.
When we'll remake warehouse module to use invoice(and built-in invoices will be removed), then all companies that bought Inventory module will receive a license for Invoice module. But just for the invoice module - not all modules that are in accounting bundle.
However work on integration of Inventory and new Invoicing module will take some time, possibly it will be available at the end of summer 2015.
So to answer your question:
If you wish to use Invoices for other needs right now - Yes you should buy Invoices right now.
If you wish to use other modules from Accounting Bundle (Timesheets, Payments, Expenses), and issue invoices for that right now - yes you should buy invoices right now.
If you don't need invoices other than warehouse, then you don't have to buy Invoices module.
Regards,
Adam
