The importance of the ACL is that it is the main stabiliser of the knee and without it fast pivoting and twisting actions become difficult and result in giving way of the knee. It is hard to get back to pivoting type sports without a good ACL. In addition repeated buckling or giving way of the knee leads to secondary damage of the other important structures of the knee – namely the smooth bearing surface (articular cartilage) and the protective shim between the surfaces (meniscus or footballers cartilage).
Once these become damaged then there is much higher risk of later problems with the knee such as pain from wear and tear arthritis. Repeated giving way of the knee is therefore not good for the knee.