I don't need them, when I first started diving with a drysuit I had floaty feet and nearly bought them. If I had I would consider that my only bad diving purchase.
Here's the deal, they work for the purpose they are designed. Whether or not you need them depends on how you dive and how much you've dove in your drysuit, plus how comfortable you are with it.
Basically they're in the same league as ankle weights IMO with the exception you're not swinging a couple pounds of lead up and down. If it comes down to a choice of weights or gaiters, go with the gaiters.