They can be used just fine. Flat pedals have a couple of advantages, such as not needing to train your ankle to get out, or being able to reposition your foot if you get hot spots. And not wearing silly looking shoes and having to walk like a duck (or scratching your floors up, in the case of a steel SPD cleat).
On the other hand, clipless allows you to pedal at higher RPM without slipping, allows you to pull up if you need a few extra watts starting up from a dead stop, and allows for on-bike calf stretches.
I actually find clipping into one-sided road pedals more annoying than clipping out. Dual sided SPD mountain pedals are very easy to get in and out of.