Multiple reasons, inter alia:
1) Locks you into spending at Woolworths in order to get the benefit from the fee paid.
2) Increased spending with Woolworths always lessens the bonus point offerings.
3) 10% discount is already available from Woolworths if you have an insurance product etc.
4) You'd get better offerings for bonus points, which outweight the cost of this subscription product, by having multiple free accounts and cycling between those.
For me saving money often requires some effort. Paying a fee to 'save money' just doesn't seem right. I think Woolworths is targetting 'lazy', or at least time poor, people with his type of offer. If you can make it work for you, then go for it. But I can't get benefit from this product.