Because it's the only part of the kit that's actually important.
I recommend not installing the component or using other means to change Keldorn to a Cavalier in that case. I think the 1.5x dispel is still solid, personally.
actually, does dispel get capped at all?
It doesn't; see the EDIT.
Keldorn will eventually fight any Evil character.
He won't. He fights Edwin and Viconia. That's half of the available evil characters in the game; he has no qualms whatsoever with Korgan and Sarevok. Keldorn also won't attack an Evil Player Character solely on the basis of what their character sheet says. This was explicitly in the post you're quoting. In the Enhanced Edition, Keldorn doesn't even want to kill Dorn if you join them after doing Dorn's quests that involve killing righteous people; Keldorn only leaves the party if you accept Dorn's quests, but not afterwards.
Those who want to romance Viconia would do well to either keep Keldorn out of the party or slap a helm of opposite alignment on Viconia.
There's no Helmet of Opposite Alignment in BG2. Furthermore, Keldorn's conflict with Viconia isn't triggered by any alignment flag, it's just hardcoded into the game as a vitriolic interaction between these two characters. Changing Viconia's alignment wouldn't do anything.