never have, cant see why?

maybe share why you are asking the question

I don't bother. You can lock the handset remotely, and you can bar the account with a phone call or the self-care web site.

I do for one reason removal of SIM card into another handset and get MFA SMS

Handset lock does not lock down the SIM card

If you set a SIM PIN

• you will have to enter 2 PIN codes to unlock your modern mobile phone, 1st for the SIM, then for the mobile OS

   

  • back in the day, when it was the "only" thing stopping ppl from accessing your phone, it made more sense
  • I do hope you set a PIN for your mobile OS?
• If you set a SIM PIN, cellular services won't work until the SIM is unlocked

   

  • No receiving of phone calls after a reboot until the SIM is unlocked. With no SIM PIN, but with an OS PIN, you'll be able to receive phone calls, it just can't access the address book to display the name of the caller
  • No receiving of SMS after a reboot until the SIM is unlocked
  • Both above scenarios are possible with modern phones that reboot overnight after an OS update

How concerned are you about loosing access to your SIM? Be that through theft, SIM cloning or your own forgetfulness?

I vaguely recall back at my time doing account management for a telco, that SIM locking was more important for when you were overseas. 

I dealt with an account where the phone was stolen, no SIM pin. Sim Card taken out and was used as a number for those prepay calling cards and our customer racked up a five figure bill in about twelve hours.

I don't know how it works anymore (or even at the time) to be honest. 

I've thought about doing this in the past when travelling, but it was too far down the list of priorities.

@mudguard. Would that have happened if the account had a resonably low spending limit?