A bit difficult to insist that the Constellation would be an Excelsior backup plan in that case, I guess.
Depends on the mission role. Even the Enterprise-D doesn't race around at warp 9 all the time. The four nacelles of the Constellation suggests endurance and stamina to me, not speed per se. A lot of fans have speculated it means the Stargazer has basically duplicated drive systems - two sets of nacelles, two impulse engines, two warp cores - the idea being you max out one, then switch to the other without interruption whilst the first one rests. Seems ideally suited to a deep space role to me.
The Enterprise-D might be able to hit warp 9.6 for 12 hours, but maybe the Stargazer could fly at warp 7 almost indefinitely?