The only time i would say to not use a solid as possible ground plane is when you have a physically large board, >15x15cm and cannot flood fill the top side.
I am assuming that you are referring to 2 layer board? I have just made a 30cm x 30cm multilayer board with no warpage issues. I did have to make sure that the layers were balenced, with a 0V and a power plane across almost the entire PCB. The only part that wasn't was the front end were there are large spacing requirements.
From experiance, I would say that having one contiguous 0V plane is the easiest single thing to help pass EMC requirements, so the only time for not having one is down to safety clearances. The single plane allows a low impedance path for the currents flowing in the circuit. This reduces the loop area and loweres both stray emissions and the loop area that is susceptable to noise. I have seen a two layer board that had an LED lighting at a certain frequency. The signal for the transistor that controlled the LED went all over the board. Measuring the loop of the signal
and the return path produced a value of the 1/4 wavelength of the signal so what was happening was the this loop picked up enough energy to turn on the transistor and power LED.
What really annoys me is the lazy way datasheets just pull up ideas that were good for 2 layer boards where there was no ground plane. I am refering to spliting 0V into analogue and digital on simple circuits. While this works if the designer knows EXACTLY where all the currents flow then it will work. However it only takes one IC connected to a different supply to cause all sorts of EMC problems (and a return of the unit marked "Failed").