Interesting question. If the membrane were fully permeable to all substances, then anything could enter the cell. This would upset the balance between the cell's contents and the outside environment. There is only so much ability to store substances and utilize substances, therefore, the cell would not be able to maintain homeostasis. For example, the cell membrane is selectively permeable and allows substances to cross the membrane if there is a concentration gradient, and if those substances are small enough to cross the membrane. Sometimes, the proteins in the cell membrane help materials enter the cell. However, this is all regulated at a particular rate. If the membrane was fully permeable, substances would enter constantly and the cell couldn't use or handle everything coming in, resulting in the death of the cell.