Well, complementary attributes stack much better with each other than a single attribute stacks with itself. 10 AD, 10% AS, and 10% Critical strikes (assuming each alone would boost DPS by 10%) will collectively boost DPS by 33%, rather than the 30% you'd get from 30 AD, 30% AS, or 30% crit. This effect gets more pronounced the more attributes you add into the fold. As such, Trinity Force will end up being more effective than items that offer a large boost - but to 1 or 2 attributes instead of the ~9 Trinity Force offers.
Even with the hit to the slows and movement speed it's still the best item in the game for kiting enemies, because it grants movement speed and a slow. Which can be useful for melee champions that have kiting issues, such as Darius, as much as it can help ranged champions that want to stay at range. Especially if you're getting in a lot of 1 vs. 1 situations - which happens a lot in modes like Dominion.