Benefits of top and bottom ground pour in multilayer boards with proper ground plane layers and stackup
In high frequency microstrip components, the "cavity" (that is, the area to the side of and above the trace) can lead to a lot of radiation in certain circumstances. The ground to the side of the trace can act as a "wall" and help mitigate the radiation (especially in situations where you may want to solder a cover over a sensitive area of the circuit. You can pour ground on the top/bottom with provisions for later soldering on a shield.)

As for the impedance matching, it shouldn't really impact the signal circuit if the pour is more that 2 to 3 times the tracewidth away from the signal trace.

And, a bit of an anecdote, but I've found that the top/bottom pour helps make the copper distribution a bit more symmetrical about the middle layer which helps with flexing over a large operating temperature range (even though the risk of bending at assembly is increased, as you pointed out).