I think the difference is this.
Is your conversation with the client the actual product you are selling (management consultancy, interior design, presenting your market research, personalised fitting of a bra) - colleagues should never interrupt.
Is your conversation about something personalised and emotional. Are you showing parents round a nursery, arranging. a funeral, fitting a wedding dress? People shouldn't interrupt.
Are you there as support for customers to steer them towards some consumer choice or using the service (helping with bag drop check in, recommending a phone, getting the right size bra out of the stock room for them) - usually colleagues should not interrupt, or for a very quick query only.
Or are you there as a helpful addition to customers/service users navigating a space, where they are the service user for something else, they are getting on with it, and you have other responsibilities so that the service can run (doctor's receptionist, supermarket shelf stacker) - colleagues should try not to interrupt but if they have to it's not that rude.
If, for example, you're just in a space, like you're all there as the marshals of an event, and a race competitor comes up to you with an enquiry, I wouldn't think it so rude for another marshal to drop in quickly and say "excuse me where have the the spare folding chairs been put" or something.