Some people consider center speakers as critical to achieving accurate intelligible speech. Center speakers achieve this in varying degrees of success.
Some people dispense with center speakers. Especially if their front's are quite exotic and timbre matching centers can prove difficult if not impossible. Phantoming the center from already excellent fronts can be more satisfying than using speakers that just approximate the quality of the main speakers.
Timbre matching of speakers is important in a multi-channel set-up. A timer-matched center is more important than timber matching the fronts with your surrounds.
But you can always experiement re-incarnating your old speakers as centers. Make sure they have the same impedance and sensitivities, at least, with your fronts. If your receiver or preamp can adjust tonalities and volumes levels for each channel independently, that would be a great help, so you can adjust the center with your mains.
