I trucked a ZX-7R and ZX-10R around plenty of times in a Tacoma with the back wheel on the tailgate and a bed extender to keep it all in place. Never had an issue, and never felt like it was even close to the limit, even when getting in and out. If the tailgate was going to let go, it would do so when the bike and I were up on the ramp, putting a collective 6-700-odd lbs right out at the very edge of the tailgate and bouncing around.
Once in, the front takes most of the weight from tie-downs anyway, so the rear is probably putting 300 lbs max (including some force from tie-downs) on the tailgate at no further than about halfway out. Even with a rough road and big bumps, if a tailgate can't handle that, it's got no business being on a truck.
As for Ridgelines, I think they're awesome. The problem for them and other 'smaller' trucks is that full-size trucks cost about the same (for Tacoma and Ridgeline) or only slightly more (for Ranger).
When I was cross shopping a Ridgeline as a work truck, I could get an F-150 XLT with similar options (bar leather) for about $2,000 less, factoring in dealer discounts. It had the V6 EcoBoost, so was miles ahead in towing and hauling, and only gave up about 1-2 litres/100 km in combined driving (according to fuelly). The cab was way, way bigger as a crew, too. Sacrifices were driving dynamics and road manners, and it's a pain to park in downtown lots.
If I didn't use the truck for work, the Ridgeline would be a lot more tempting, but full-size trucks are so competitively priced that it's tough to compete with something smaller. That's my theory for why light trucks have all but disappeared...
