Timbre matching is basically ensuring identical frequency response CURVES. Not just frequency response. Two speakers with the same frequency response of 40hz - 20khz +/- 3db may not sound the same because the curves are not identical. Because one could dip -3db at 2khz while the other might peak at +3db at the same point. But they are technicaly of the same F.R. operating within the same variation range.
Timbre matching is essential when we have a multi-channel/speaker set-up. Not just for home theater but for multi-channel SACD and DVD-A and even more so on 5-channel, 7-channel or multi-channel stereo modes.
For HT, you'd expect a jet plane going from front to back to sound like a jet engine from front to back and not become a bi-plane when it reaches the back. Similarly, horses racing on sand dunes shouldn't sound like horses racing on wooden floors when they reach the back or any part of the set-up.
In multi-channel stereo music, a timbre mismatch among speakers can be disquieting. A solo singer can appear to be two or more especially if the back imparts a totally different sonic character to the voice compared to the front.
Maybe not so critical with DVD-A and SACD especially if the back channels have discreet instumentation totally different from the front. Still, the recording engineers mixed the multi-channel instruments on the assumption that all speakers are identical. And as audiophiles, we'd prefer to listen to how the engineers mixed the music without chaning any variables.
Timbre-matched speakers don't have to share the same dispersion character. One can be a dipole, bipole or uni-pole or whatever, provided they all share the same FR curves on and off axis.