There is a whole world of possibility for adult stem cells, however, pluripotent stem cells, such a blood cell precursors, are not totipotent like a blastocyst stem cell. Basically, this means that the blood precursor does NOT have the ability to turn into EVERY cell type, but can turn in many. The blastocyst cell, on the other hand, is still in at a stage where it can differentiate into any cell type, and this is why they are important.
This point is clearly illustrated by this case. Sickle cell anemia occurs because of a genetic substitution where the codon for a valine is switched for a glutimate. This causes the Hb molecule to be bend. Unfortunately, this also causes the sickled appearance of the cells. If one could get an ALREADY differentiated blood cell procursor and placed it in the right place, as in this case, then of course healthy blood cells could be made. However, this differentiated blood precursor cell from the embilical cord, could not make, a neural cell because it is too far differentiated, and this is why science is still looking at 128 day or less stem cells.