Well you had at least three classes running around with the same basic parts: Constitution (refit), Miranda, and Soyuz.I always figured the Constellation-class were easier and cheaper to build than the TMP era Enterprise-class cruisers while using many of the same parts. The Constellations look to be of simpler construction and less elaborate design--hell, it's really just a beefed up saucer section with the stuff normally in the support hull of a Connie shoehorned into the Constellation's main section. The four nacelles are not exact replicas of a Connie's, but serve to provide similar performance in a quad arrangement.
The Enterprise and Constitution classes were the top-of-the-line while the Constellations could have been more workhorses to perform many of the same functions while more of them could be built for less investment per unit.
The variants of the Constellations we saw could have been science survey specific variants bulging with extra sensor systems. Meanwhile there could well have been variants that were cleaner and more streamlined looking akin to the familiar Constitution-class.
It's entirely possible that the Constellation was a "budget" ship. A ship meant to save on retooling and updating Starbases and shipyards by using Off the shelf parts. With the known galaxy in a state of political flux, it might have been seen as a "stop gap" ship that wasn't meant to replace the frontline ships like the Constitution completely but allow the fleet to draw down the older spaceframes till a more suitable class (Excelsior) could be put in the field.
I get the feeling from ST II (and I can't explain why) that around that time deep-range exploration wasn't at the top of Starfleet's list at the moment.
I vaguely remember a few lines from a Trek novelization (from one of the movies, I think) where a new Constellation-class USS Magellan had just left on a multi-year mission and Kirk or Sulu had sent subspace congratulations to the captain, a friend of theirs.
So I rather thought of the Constellation as a gap-filler between the Miranda and Constitution-refit designs and the Excelsior-class, using mostly Con-refit materials and design but also incorporating newer items (i.e. the sensor bulges and domes that appeared on the Constell were likely intended for the Excelsior, but reduced and streamlined to be more asthetically pleasing in the latter design after thorough testing on the Constells).
The quad-nacelle arrangement I thought of as all four working together to provide longer duration cruise times, though not necessarily faster warp speeds, rather than one pair working/one pair back-up. Hence the slightly different nacelle design, to accomodate for quad operation.
I also pictured the Constellation-class as being designed for mission durations past five years, such as ten or twelve, going farther out to accomodate the grown of the Federation and the expansion of explored space. In the twenty-plus year gap between the time of TOS and the Trek movies, I would imagine the Federation and explored space has greatly expanded, requiring more than five years for a mission. No point going out to the limits of known space than having to return to change out the crew after their five years.
And if I remember correctly (I'm sure one of you will share with us the actual statistic involved), the Constellation primary hull has about 3x the volume of a Constitution-refit-class primary hull. If indeed the Constell was designed for missions greater in duration than five years, then they would need plenty of space for consumables, shuttles, replacement parts, other apparatus as there would be few starbases where the Constells were heading.
We don't know if there were many Constellations built or only a few; I can't imagine there were too many built by the time the events of ST6 inspired an intelligent drawdown of Starfleet, but the design was decent enough that examples survived to still be in-service during the time of TNG.
An aside...what was the Constell serving in the sensor net fleet in TNG's "Redemption"? Is there a listing of the ships that served in Picard's task force?