Anything is viable if you use it correctly, and it's no different with Tropius. He can use it if he wants to. I WOULD recommend getting Solarbeam over Silver Wind, because loads of things can KO it if you're not careful, and shaky stat boosts with a move with low power isn't going to be really worth it on Tropius if you're lucky (although I will admit that a boosted tropius is a monster, so you could put it over Giga Drain too). I'd recommend focus sash over expert belt too, for the earlier reason aswell.
Another reason why Tropius is viable, especially in this case, is because it's a Chlorophyll user. However, I would advise being careful how you switch between sun and rain on your team, because enemy rain teams will walk all over Tropius with Ice Beams (ironically D-nite too). If you're up against a rain team, I'd recommend prioritising sun to utilise Tropius, and using your rain users as counters instead.
Did you know there was a Plusle Minun combo sweeping OU about half a year ago? Yeah. Less powerful pokemon in OU are, most of the time, in not-serious teams, but it doesn't mean they can't be used.