This is all very well, and I feel Hecabe's grief as strongly as the next person, but what strikes me is that nobody ever wastes any sympathy on Cassandra. On Hector, yes, on Priam, plenty. And even though Paris generally comes in for a good number of "served him right" comments, at least he comes in for something.
Part of the problem is that Cassandra is such an insufferable know-it-all. If you had been the king who had his curtain drawn in the dead of night to be told half his Troy was burned, would you have been thrilled to have a daughter raising her hands and eyes to heaven as though to ask Apollo what he would have done if surrounded by such idiots? (Or, worse, turning back to shout, "I told you so!" as she was dragged in chains to the enemy ship?)
Cassandra's real tragic flaw is that she doesn't have one. She was the Seer fated to be disbelieved; had this been because of some girlish crime you could have winked slyly and said, "Ah, the girl's one of us!" But this was because she was so good and pure and virtuous that she refused even Apollo's advances, and commendable though this might be, it does give you the feeling that her shade is watching you from Hades with an air of pious disapproval. You can't really feel sorry for someone like that.
That, I think, is why so many people have a bit of sneaking sympathy for Clytemnestra. If your husband brought home a younger wife and she flaunted her nubile charms in the face of your mature but true affection (I'll do Clyemnestra a kindness and forget Aegisthus for the moment) you could at least have the satisfaction of calling her names. If she looked at you with large sorrowful eyes that saw every frivolity and transgression you had committed (here we bring Aegisthus back) while her city was being reduced to smoking ruins, and in all likelihood saw all the frivolities and transgressions you would commit in all the rest of your life, it would probably infuriate you even more than the bare fact of her existence.
The moral of the story? If you do know what's inside that nice big horse that the Greeks just happen to have left on the beach, don't tell people about it. They won't believe you, and when the warriors burst out of it and attack they'll only be annoyed with you for having been right.