Years ago I recall reading in every publication that Timbre matching was absolutely critical. In recent years it seems the pendulum has swung the other way. I've seen many articles in traditional magazines that have said Timbre matching is not as critical. I do think I agree though I'm sure most people on this forum would disagree with me, and to tell the truth I feel no need to defend my position to others. I will say that from my own experience I have owned several different matched sets of home theater speakers over the years. To my ears none of these systems ever had all 5 speakers perfectly match. Two of these systems (one was embarassingly cheap since this was my first purchase, the other was a THX certified system) had LCRs for the front three channels. The pink noise used for calibration always brought a different sound from each speaker because of placement variations. The cheapest of the two had the exact same speaker for all 5 channels. The surrounds were mounted to the side and up high on the wall. Of course this location by default sounded much different than the fronts. The THX system had small surrounds that had different drivers from the fronts. That, coupled with a smaller enclosure and placement on the wall ensured they were never going to match perfectly.
My two most recent systems had two towers, one smaller center lying on its side and surrounds in enclosures much smaller and shaped very differently from the rest. Both of these were matched systems, but again, due to placement variations, enclosure size, design variations, and driver complement, none of these speakers sound exactly the same. Though when I played movies or music I rarely noticed any issues, except from the surrounds.
I'm not saying that timbre matching doesn't matter to the point where someone could buy 5 different speakers, but I don't think it's ultra critical.