You have asked an excellent question!
Most surgeons in the United States use smooth, round breast implants. Why?
First of all, it is important to realize the final shape of the augmented breast depends on several factors.
First, if you start with nice looking breasts, the chance of achieving nice looking augmented breasts are much higher.
The shape of the operated breast is very technique dependent. One can achieve bad results with either style implant if the implant pocket is too tight or too big, and the implant is too large or too small for that patient. Therefore, do not assume the tear-drop implant will automatically give you a better result.
There are three main disadvantage with the tear-drop implants. First, since the implant is flatter on top than the bottom, if the implant should rotate (and they usually do), then secondary surgery is needed to reposition the implant.
The implant company has made the surface of the tear-drop implant textured (rough to the touch), in order to minimize the chance of rotation. However, the downside of the rough surface is that the breast tissue will "stick" to the implant, which will increase the chance of rippling (wave-like motion under the breast skin).
The third disadvantage of the anatomical implants is the cost. These implants cost more due to manufacturing expenses. In my opinion, the theoretical advantages of the tear-drop implants are not worth the potential risks involved.