I use scripts more often than I don't. It is the most useful speech tool I have.
In your type of scenario, I'd either say what I had planned--or, more likely, stare or make some kind of unconscious grunting noise. (Finally getting better about that when a colleague I respect once asked if I was mad at her after I did this. All I was able to say at the time was, "no". Later, I explained to her that I was, in fact, frustrated, but not at her....at my inability to communicate what I wanted. I am thankful that she said this to me, because it NEVER occurred to me that anyone would think I was upset with them when I can't speak well enough to get something across.)
I have 3-4 people I must speak to every afternoon at work. It is one of the most anxiety-producing parts of my job because of the speech flexibility expected. To prepare, each day, I decide what I will say and rehearse. It works about 75% of the time. The rest of the time...it's just embarrassing.
I think I usually let clerk/waiter/cashier speak go. I have very frequently told food service people, "you too" to "enjoy your meal". I'm sure I'm not the only one who ever does that. I just laugh about it when I notice.
For KB8CWB - I don't know why anyone else does it, but if can tell you why *I* do.
I cannot think quickly enough or with enough words at my disposal to answer otherwise. I once started to draw out a map of how my brain seems to access spoken language. The first available, and easiest to use, is scripts. If I didn't use them, I would barely speak at all.
_________________
So you know who just said that:
I am female, I am married
I have two children (one AS and one NT)
I have been diagnosed with Aspergers and MERLD
I have significant chronic medical conditions as well