So, there are reasons to use Postmates (owned by Uber) that need not be discussed here.
Uber did merge the logins so if you are an UberOne member, Postmates Unlimited is auto available when you use the Postmates-side website. What I did not expect because I didn't see it disclosed anywhere is Postmates-led charges (which uses the same Uber Wallet tools) are credited at 10% back like Uber charges by Capital One.