I disagree, Imho they're generally not very good. The big base is a big disadvantage because you can't get them everywhere and the fact that you can't support them also reduces their value, because if you have 1 attack it means you're often dead because you've been charged by 1 or 2 supported models, often with S4... oops.
The only really effective cavalry are Serpent Riders and Haradrim Riders because they're dead cheap for lances and Serpents have F4 which makes a difference when fighting F4 good troops. They're also in an army that can afford having them without losing its main strength - bowfire + numbers. I would've taken Serpent Riders this year but had to settle for Harad Riders because I don't own the models and forgot to borrow earlier.
The "Need for Speed" in Legions of Middle Earth scenarios is a complete myth - it's not true that your army needs to be fast, but as you can see the winning Good armies of the past 6 GTs included what, 8 mounted models in total? On the other hand, other than 2010's winner every one of them had a Wizard that made a big difference - both Gandalf and Saruman are viable choices. Yes, sometimes it's good to get somewhere first, but if you don't have cavalry then you can always move someone away from objective with your Saruman or Shadow Lord
Take and Hold is the only scenario which really requires speed because often the person who gets to the middle first is the one who ends up winning (unless you kill or tie their heroes up and break them, since the break rule says that you only count as broken at the beginning of the turn when you have below 50% models - essentially that means that you always have to play 1 turn with courage tests after you reduced your enemy below 50%, and a lot of the time that means that they're fleeing if their heroes aren't helping with Stand Fast, so it doesn't matter if they were there first).