[mrb]

There are many poor design decisions in the cellphone infrastructure, but the SIM card is probably one of its best pieces.

Broken phone? Pop the SIM card into another phone, and you can immediately make and receive calls & texts on the new phone using your phone number.

If you had no SIM card, how would you authenticate yourself to the cell network (that's what the SIM card does)? Going online and then providing a username/password? This would be horrible security-wise as we all know people are terrible at picking secure unique passwords. So hackers could try to guess your password, then they would use your account, receives your calls & texts, and they could steal your cell data, causing you to receive large cellphone bills, etc. A total nightmare.