If a bartender at a sluggish restaurant is scheduled monday-wednesday, instead of his usual fri-sunday, he's probably screwed and making a fraction of what he normally does.
The main problem with the DD model is that they're asking people to decide a tip before being provided a service. That's unfair to the customer and driver. As far as paying a decent wage before that, DD could provide a mile cost reimbursement as well as one for the time spent on the order. Paying the driver a blanket fee for every order they deliver rather than one based on time and millage is not sustainable. I understand that's much how the model used to work, but the whole thing about tip culture is rewarding people who do the little things to make the experience worth it for you having to pay extra. As a customer of DD you're only going to be aware of things after you've received your food.
Also, if a bartender is not getting the best shifts, that may be because the bar cycles their shifts or they aren't as good, or maybe Wednesday is a busier night and they only staff one bartender so that person may end up doing better than having to split a bar with several others. That said, if it's busier on a Saturday and the bartenders are getting hammered, why should they have to subsidize the bartender who isn't there?
That said, the biggest problem with most service type employees right now, and this sadly gets away from DD, but is the amount of hours they are being given to work. I know many people who only get 12-18 hours a week at their service job (and even worse, you can be sent home early or even called off if it's slow), and even in CA where minimum wage is $14.25, one cannot pay rent let alone earn a living or even hope to have or provide for a family.
You can still have tips but it shifts from basically every time to instances of truly exceptional performance.