I think one reason is to establish a dominant Stargate if two are in a region. If you're using unique identifiers for Stargates, it's more likely you might get into a situation where two in close proximity are activated at once, and I imagine that having multiple wormholes activated so close together is very bad. By making the coordinates be location based, an incoming wormhole will always try to establish with a dominant gate and disengage if it's busy, rather than trying to establish a wormhole with another gate in the region and have havoc ensue.
One consequence of this (established before the earlier reason by the writers needing a good script) is that Goa'uld tactics sometimes require moving Stargates and therefore they would require location-based coordinates. Consider the following:
In Season 2's The Serpent's Lair, Apophis arrives with a Stargate with his Ha'tak and dials out to a planet when he arrives, thus preventing Earth from using a Stargate to flee. Presumably this is done in most combats. Since there is a "dominant" Stargate in a location, it makes more sense to just refer to the location you are in, since that's the time when it's going to be advantageous in battle and it wouldn't matter which Stargate you bring with you (or heck, you could have multiple Stargates in a fleet of ships).
It also makes for a more effective base planet: put your ship with a Stargate at your main base planet, everybody rendezvous by gating to your home planet, you go off to fight your enemy, and if things go bad, you all gate home. This ensures that everybody is going to the same place and they only have to know one address, not two.
Also Ra in the alternate Moebius timeline takes the Stargate with him when the humans rebel. Either he put it on his ship or he dropped it on another planet. Having Stargate-based coordinates would mean that a cartouche describing Earth will still point it it, even if some Goa'uld years later drops a different Stargate there (which apparently happened with Yu and Sokar picking up followers at various points in history).
This also means that you don't find an outdated cartouche and then wind up on a frozen planet on the other side of the galaxy because it was describing Earth. After all, you're more interested in your destination, not which ring you come out of.
Also, it'd mean that whenever the SGC swapped between the Antarctic gate and the Giza gate, they'd have to learn a new set of coordinates (or in the episodes where they suddenly switched, like 48 Hours, teams wouldn't be able to dial back to Earth because they wouldn't know that a switch occurred).
There are benefits in that you don't need to worry about stellar drift, but ultimately, that's a very long term benefit. The short term benefits of a location-based system seem to outweigh it.
From an out of universe perspective, the first time they established that gate addresses were location based and not gate based was the Season 1 episode Solitudes, which relied on the fact that there were two gates on the same planet accessed by the same address. If they had unique addresses, that episode couldn't have (easily) been written.