This is always a tricky one. Any restricted service out of Paddington will without fail reveal passengers holding tickets which aren't valid. In some cases it may be just a handful, in others it may well be a very significant number. The 18.30 and 19.00 tend to be favourites to leave from unbarriered platforms, and are on the cusp of the evening peak and off-peak restrictions; the results are inevitable. Some of these passengers are of course regulars, who know what they're doing. However, many have no idea of the situation. You could argue that the restrictions are advertised on the departure boards and it is up to the passenger to check, but personally I do feel that Guards making clear announcements, prior to departure, is plain old fashioned customer service. It also reduces the conflict, as some passengers will leave, and for those who remain and argue that the restrictions weren't made clear, the response that "we made several announcements before the train left" silences most arguments.
But, what to announce? Yes, it may be misleading to simply state "Ticket X is not valid", but it is surely equally unhelpful to say (as the boards do, in fact) "Ticket X may not be valid..." - what is an intending passenger, sat on a train due to leave in five minutes, supposed to do with that information? It's difficult to get it right; something along the lines of "Tickets X and Y from London Paddington to stations to X is perhaps as close to ideal as we're likely to get, but of course in practice there are many variables (including avoiding the hassle and checking no tickets at all...)!