I am also testing 8x8, and have encountered the same limitation / requirement.
We are more complex, but each team of users would use a 'team number' as their number (not individual DDIs).
I can see it is possible to change the outbound number presented when calling out (so all 'sales' users present the sales line, all 'finance' users present the finance line etc). I can then route these team numbers back to groups / autoattendants / call queues as needed.
So I'm assuming that you just need to make sure you have enough numbers to assign 'dummy' phone numbers to each user (and don't present them, and never publish them anywhere), and then assign your 'real' numbers to the groups / auto attendants that you actually want your customers to call into.
I think this will overcome our issue, but I think it is unnecessary to force every user to have a DDI when it is not needed (and leads to me adding multiple DDIs to my system that I don't need and will never use).