If Serpent had shown up and turned their guns on population centers, it would have meant apocalypse. For the Jaguar civilians, for sure, but for Serpent as well.
Think about it. The other Clans were horrified by Turtle Bay, leading to the Wolves declaring warship use to be out as a tactic from then on in the invasion. The other Clans had to follow suit as a result. The use of such a tactic was seen as being a grave dishonor by the other Clans, and even the Jaguars afterwards seemed mixed about it at best.
Now imagine an Inner Sphere fleet shows up and does the same thing in revenge.
...Now remember the reaction the previous time a Clan used WMD attacks- the nuclear attack on Dehra Dun. And how THAT turned out. (Yes, I know the Wolverines may not have ended up being responsible, but the Clans in 3060 don't know that.)
Rather than facing Clan Smoke Jaguar- and winning only thanks to the arrival of Bulldog (a plot point I've always hated, btw), Serpent now has probably enraged at least several Clans, if not all of them. Not only does Serpent get swallowed up by a sea of angry Clanners, it probably causes the Clans to not only never deal with the Great Refusal, it invalidates the Tukayyid Truce (Comstar ships, after all, just dishonored their faction with such a heinous act- why give them the respect of honoring the deal?).
If Serpent fires a single shot at a city with their warships, they win a temporary victory at best- and guarantee the response from the rest of the Clans, both in the Homeworlds and back in the Inner Sphere when news reaches there, will bring about the kind of horror and loss of life the 3050 invasion merely hinted at.