Studying biology or not depends on your chosen careers path in future.
Although HKU Faculty of Medicine (M.B.B.S) would recruit students without studying AL Biology, it requires students without taking AL Biology must get at least Grade C (i.e. old rating system) in HKCEE exam.
As you have said in mail, you want to become Medical Lab Technician, you must study both AL & CE biology. It would be an advanatge that if you are able to master all the concpet in AL Biology because you will spend less time on Biochemistry, Pathophysiology, Physiology and even pharmacology that you will encounter in your future degree programme.
Conversely, if you want to become Chem Lab Techician. You are more apt to take AL Chemsitry only!
Personally, Studying AL Biology would be one of comeptitive advantages in future if you want to do a Medical, Nursing, Biotech, Pharmacy and other health allied programme.