Is it recommended to transplant hair if the donor area is very weak? How effective is the Stem Cell for regrowth or transplant?

If you don't have good donor hair, a hair transplant surgery would not work.  There is no such thing as stem cell therapy in hair transplant surgery.  (at least not in 2016)

Technically you can have a transplant despite a weak donor area, it's just not advised because the back of your head will look sparse or even patchy afterward and the scars may be visible, because all the hair has been moved to the top of your head.
There isn't any method of using stem cells to make hair grow currently.

In order to determine your donor capacity a consultation with an experienced surgeon is necessary which will further give you an accurate estimate of the number of grafts necessary considering your calculated density, hair caliber, coverage value and number of hairs per follicle. If your estimated coverage value is considered to be sufficient, an extraction of a certain amount may be carried out. The donor capacity shall be considered as sufficient if it is possible to cover the receding and balding areas with the extracted amount in a satisfactory way.

If you have a condition called Diffuse Unpatterned Alopecia (DUPA) that is a contraindication for any hair transplant surgery.