The majority of the time you can get a feel for a weapon without a riven, but they fundamentally change a handful of weapons. Like with a really strong Opticor riven you can almost triple your DPS and fix the fire rate issue by using partially-charged shots for virtually everything outside of Grineer heavies in S3 armor-enhanced sorties. Sicarus Prime can be ridiculously powerful with e.g. a dual crit+damage riven, but -90%+ recoil gives it Lex Prime group sizes with significantly better DPS and armor stripping plus a much lower time between shots for full accuracy. Kohm with a 120% SC+preferably MS/FR or DMG riven is a completely different gun since you can get 100% SC with just three 60/60 elementals and give yourself much more room for damage mods and better QOL than with Nano Applicator builds. That's not even mentioning Scoliac which is the new meta whip with range+CD+slide crit or similar from barely ever getting any use. Those are some of the most extreme examples, sure, but there are plenty of weapons with significantly cheaper rivens like the Buzlok or Harpak or Drakgoon that can become quite good with a nice riven. Most of the time they just make meta weapons a little bit stronger or useless weapons slightly less useless, but there are definitely exceptions.

That's not to say I like the system in its current state, but at the same time rivens aren't as optional as people make them out to be if you're a fan of particular weapons and care about the endgame. For Tigris Prime there's obviously no need, but if you really like Burston Prime and want to run it against level 100 (if we're being charitable, realistically it falls off hard before that against units that are dangerous) enemies you're gonna need to pick a riven up.