Wow, the service had declined a bit then. Still worse off now I'd say, I'm sure the direct service to Tonbridge would be useful to some people.
I don't have a relevant timetable to hand but I think that the superior power of the Turbos means that the service isn't slower today with the extra stops at Blackwater and Reigate. I don't get the impression that there is much off-peak travel across Redhill to Tonbridge/Kent although the current timetable doesn't help.
Going back to 2002, one of the most useful Voyager trains was a service that originated at Brighton at 2023, called Haywards Heath, Gatwick Airport, Redhill at 2115 and was in Reading at 2201. This was starting to get good loadings by the time the service was cut back for Operation Princess. I seem to recall that an afternoon peak service northbound from Guildford loaded well with cross-Reading travellers.
One of the problems with the Virgin trains over the North Downs Line was that they couldn't be timed in such a way that they would reliably present themselves at Reading from the north in time to go to Guildford in front of the, then, xx07 stopping service. They consequently had pathing allowance at Guildford or would have waiting time on the signal at Blackwater awaiting the section to clear.
Saturday mornings used to see a fast turbo path Reading-Guildford-Redhill-Gatwick each hour between 0700 and 0900. These went a few years ago but if you want to judge whether Gatwick is a big traffic generator, you only have to compare loadings on the trains that go there over with the stopping service that starts at Redhill. West of Guildford, the distinction is less once the flows into and out of Guildford and Reading are considered.