Each model is different and each brings a different "look" to a shoot. If you're shooting the same thing, then sure, it's great. But if you're shooting variety, then it's good to have different models for different situations. Some examples -
If you're shooting traditional pinup images, you wouldn't want an Asian or Indian woman to be the model. How many classic pinup images have you seen (classic being WWII traditional type images) of Asian or Indian women?
If you're doing a classical Geisha shoot, then you don't want a Caucasian blond hair blue eyed woman for the shoot.
If you're shooting a fashion shoot with "little black dress" as the theme, you don't want your 250lb Aunt Martha out busting through the dress in her tallest heals.
If you're doing a biker shoot, you shouldn't be using your neighbors 14 year old son that looks like he belongs in a boy band. Go out and hire a guy with long hair and sleeve tattoos that looks like he belongs on a Harley.
I don't want to sound chauvanistic (I probably do) but we're talking about commercial stock here. That being said, once you build up a group of people that you work with, and you feel comfortable working with each other and can communicate effectively, then yes, the same model does help a bit.