I believe the LD50 was a useful test at the time at which it was developed. It served the scientific community well for many years. However, advances in toxicology which have occurred since its introduction now allow us to better characterize toxicity without using this test which is now an anachronism.
Rall, David P., "Letter from David P. Rall to Henry Spira" (1983). Letters and Correspondence. 4.