You can jam in a bigger OLD and it's ok, but that doesn't mean its easy to do so. The frame fights you every time. Eventually I gave up and got the correct OLD. The frame was sooo happy and so was I. With a smaller OLD, when you tighten the axle nuts you will think the wheel is snug in the dropout but it won't be! A quick release type of clamp may not care and you might get used to having to use more force than usual to get things properly tight, but axle nuts, I don't know. But all that aside... coaster brakes are usually the only brake on bikes so equipped. For the usual application: juvenile, special needs, utility, etc. having just a rear brake is not seen as a huge liability. With higher speeds and use on public roads... I don't know.

I would sooner use no rear brake at all, and leave all the stopping to a front brake of any technology than deal with the hassle of a combination front brake and coaster brake. A front brake only setup works very, very well, and I have unintentionally used this configuration a few times when a rear brake was out of service and I was too lazy to fix it right away. I miss the rear brake when maneuvering at low speed and I want to do something with my left hand like activate the garage door opener or signal a left turn or something while keeping control of speed at the same time. But for pure deceleration, there is not an absolute need for a rear brake, and coasters are usually heavy, hard to modulate and of relatively poor quality because of their presumed application and market.