A retainer doesn't necessarily cost much. We are not talking a whole bunch of money, on average less than $50. Often lawyers will not cash that retainer, but will hold onto the check or authorization to pay and will cash it if you ever need their services. My aunt is kept on a more costly retainer for clients such as bounty hunters who, in the course of their regular job, can and do find themselves in shady areas of the law and come into contact with law enforcement, where a lawyer's connections are certainly helpful. Basically, the retainer is not much for some regular person who just wants some insurance against the unlikely event. In this world, money talks! A lawyer who does not know you from Jack will work for you if they decide to take you on as a client, but it is guaranteed that that person WILL go the extra mile for you if he/she knows you personally.
By the extra mile, I mean that they will call their contacts at 2 in the morning to get you out of jail, instead of waiting until the morning. When you hire a lawyer for a prickly case such as a self-defense shooting, you are hiring their connections as much as you are hiring them. If your lawyer is not family or a close friend, they will NOT rattle those connections for you at 2 in the morning if they haven't been paid yet, if you think that they will, then you are dreaming. If they have those connections, they do not NEED to jump for someone they do not know. If they have a retainer check from you, they will be more willing to make those calls, because they already know that you are good for it, and MONEY TALKS. Their contacts will not like being woken up at 2 in the morning just to help them get someone they do not know, someone who just shot a person, out of jail. That lawyer will owe their contacts for that service, and you will have to pay for that debt.
Personally, I would rather pay $50 now to make sure that if I ever need to shoot someone and get arrested for it, I'm not going to be spending a night in jail. I imagine that after being attacked, jail is the last place that I will be wanting to spend the night. A good, well connected lawyer can make sure that even after being arrested, you can get out on your own recognizance without having to spend a night in jail and/or coughing up bail, this is NOT how I want to spend the first hours after being involved in a shooting.
Is this service worth a little retainer? In my mind, absolutely. Also, I live in CA, Southern California to be exact, where a person is much more likely to be arrested in connection with a self-defense shooting. Even if they think that it was justified, they seem to prefer to arrest now and ask questions later, they'll sort it all out down at the precinct. This makes it doubly important to make sure that you have a good lawyer on your side if you decide to bring a gun into the home to protect yourself.