Will the information still be useful if I often forget to log the kicks or contractions?

Yes. Assume you're talking about the practice of monitoring "fetal movement" as a proxy for fetal well-being. The premise is that babies who move a lot are healthy babies, and babies who don't move may be "asking for help". Babies in utero sleep in 90-120 minute cycles. If lack of movement persists much longer than that, call your obstetrician or go to l&d. 99% of the time, he/she is fine.