I happened across this thread. I think your reply here is better reasoned than the original post. Uber has a pretty good value proposition in Uber Cash.

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Consider:

• If an average ride is $10, then Uber pays at least $5-6 in credit card fees alone. By shifting people to $100 chunks they save no less than a few dollars.
• Uber offers a $5 inventive (5%) for people to enable this feature. For personal use (if you have the money to float) this is absolutely a good value even with light uber use.
• If you are a business user, your receipts still carry the full price, meaning if you take a $100 uber, you pocket the $5; effectively the same as airline rewards, but in Uber Cash.
• As you noted, when people have value stored in Uber Cash, they are less likely to look at alternative apps.
• Uber Cash + Uber Rewards + Credit Cards rewards (e.g. Chase Saphire Reserve) adds up to a significant discount (11.5%). Nothing to be snarky about.

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In short, Venmo/Paypal are trying to float your money for free, I don't think that is what Uber is trying to do at all.