Why would an eclipsing binary be moer valuable than anordinary spectroscopic binary?

Question

in accurately determining stellar parameters?

Answer 1

Because when one star eclipsed its companion you would have only one spectrum showing. Therefore you could deduce much more detail about the individual stars.

Answer 2

Further to the previous answer; if it's eclipsing, you know the plane of the orbit, so the Doppler shift of the spectra accurately measures the orbital velocities (in a non-eclipsing binary, the orbital velocity is greater than the Doppler measurement by an unknown factor).

In addition, the diameters of the two stars can be accurately measured because the orbital velocity is known and you can find out from the light curve how long one star takes to pass the other.