Here is just some of what I came up with while researching last years Aussie Trials Titles programme from the South Aussie side of things.
Chris Holmes - trials to Enduro - 1st SA rider to Medal at an Aussie Enduro championships, a 24 Hour Reliability Trial win - and back to trials to win the 1st Post Classic Title in 1994
Steve Martin - Aussie Junior Champ to World Superbikes & World Endurance Championship
Glen Richards - SA Junior Champ to ARRC Champ to British Superstock Champ & World Endurance Championship
David Holt - SA solos & sidecars to 3 time Aussie Enduro Champion & ISDE Gold Medallist
David Maunuel - Aussie Junior Champ to multi MX sidecar Champ, multi Finke Desert Race sidecar wins and 6 SA 24 Hour Reliabitlity Trial sidecar wins
Shane Diener - SA juniors to Aussie Enduro Champ, ISDE Gold Medallist, Aussie Vintage MX Champ, 9 24 Hour Reliability Trial wins and Aussie Safari Stage placegetter.
Phil Medson - trials to Speedway, competing in international competitions
Andy Haydon - trials to Supercross & MX Championships, multi 24 Hour reliability Trial wins, and, a 3rd outright in the Dakar Rally.
Of course in the good ole days it was a lot more common to ride more than one facet and a lot of riders were multi facet. From the Keyneton MCC side of things there were many including;
Dennis Weichert - trials, enduro and reliability trials
Ken Warnest - trials, reliability trials and road racing
Jim Warnest - as above
Bob Petts - trials, reliability trilas and scrambles (mx)
Geoff Warnest - as above
Max Lee - as above
Ian Zander - trials and reliability trials
Trevor Diener - as above
Philip & Roger Holmes - as above
Geoff Barratt & Laurie Williams - as above
Just to name a few.
The bottom line - there is little doubt trials is the best grounding for all facets - it can help every facet in some way.
Sidecars - its betta to have a swinger than fly solo!