TBH clone onlys are normally kept for a specific reason, the taste or smell or growing characteristics etc
I.E. Exodus Cheese.....Was found in a pack of Skunk #1 seeds and was kept solely because it tasted and smelt like cheese and is incredibly potent, in its clone only form it is a good yielding, very potent smoke, whereas just recently it has been selfed i believe and seeds made and none of the seed versions/crosses have anywhere near the potency or depth of taste/smell.
So basically....clone onlys are kept to preserve a certain set of genetics because when they are selfed and crossed they lose something, whether it be potency/taste/smell etc