Hard drive access speeds can cause stuttering in certain games if they are too slow. A perfect example would be World of Warcraft. Many people experience smooth gameplay when questing by themselves...but in large raids, busy cities or jam packed battlegrounds they complain of a lag effect. They add memory, upgrade their CPU or graphics to no avail. This is because the problem here is related to access times. While the game world and 3d effects the program can predict to happen will already be loaded into memory, the objects, textures and effects it can not predict will have to be pulled from the drive. These unique and unexpected objects come most commonly from the unique textures that make up other character's appearance and activity.
Games like first person shooters are able to load pretty much every required rendering file into memory since the number of players is often very limited and the number of simultaneous effects can be predicted quite easily.
Again, it's about what you play...there's no blanket answer to the question of hard drive stutter or lag. But for my mmorpg playing customers, I will continue to suggest solid state drives (and if mechanical hdd units get any more expensive i'll suggest them to everyone.)
My first post here...try not to flame me too hard.