Isengard does have good pikes, but it takes them too long until they get there. Going for Knights vs Isengard in the early game wins you the match, unless you make some terrible mistakes. Isengard can not, under no circumstance, counter Knights until they have pikes - which means that, if used correctly, you'll have the entire map and even a considerable infantry army by the time he has the pikes to stop you from destroying every single external building and unit Isengard has. You should have one or even both outposts and siege (and the economy to get upgrades at the same time) if Isengard doesn't get Wargriders, but even if there are Wargriders, Knights just mop the floor with them. And this is all mostly talking about harassment, not even about their great trample and melee damage that allow you to pick off archers in bigger battles or finish off fleeing swords or completely wreck any isolated unit or hero, which takes so much mapcontrol off your opponent that it's just glorious for you. So yeah, not really sure what you're talking about there, Kryptik^^
You can make Knights work in pretty much every matchup. In my experience it's hardest to do against dwarves, followed by Gondor, but they still have their uses their as described above. If you get just two batallions of Knights, it forces your opponent to get at least four pikes, depending on the map, otherwise either his army or his flanks are unguarded. Having the initiative and forcing your opponent to react to what you're doing is a great thing in itself, but in this case it has the added benefit that pikes are more expensive than swords (ESPECIALLY useful against Mordor). And if he goes pikes only, he can't stop neither your infantry nor your heroes.
Now to Dol Amroth: I have to point out here that I go for the standard outpost nearly everytime. I just like the extra versatility and it gives me siege without having to demolish a building in my base. Anyway: Dol Amroth cavalry is pretty much the heroic version of Gondor Knights. Since I get Knights nearly all the time, I don't get the DA cavalry at all, because if the match isn't decided at that point my oppenent usually has enough pikes to counter my Knights, so those wouldn't make the difference.
The infantry is quite nice when you have a large standing infantry army. They are good swords themselves, but the leadership is the best thing about them, so three or four units are usually all I get to support my army.