Solar party is still viable on some of the maps that are too annoying to nuke through normally, such as high def (damage will still get reduced), negate, or gimmicks like in a few Nushi maps where damage is reduced. It also allows you to have the quickest farming times on some maps, which is also the most effective way to farm ampies.
Here's an example of solar usage at Aqua Shadow Higanbana EX Destruction. Normally the main boss would receive 1 damage unless the two shadows in the same battles are dead, but instead solar setup allow you to skip the mechanic as well as the previous two node battles.
https://streamable.com/4vao0

Of course, the overall damage is nothing significant compared to crit setups, but there's a lot of playing room opened up since the introduction of RGs like Evasion+Shine drop solar setup.
Flower memories also bumped the solar effective value significantly too as each memory are treated as it's own multiplier (for example, multiplied 1.1x for a total of five times) rather than added up together to like 50%. This means if they release a 6* or make fully LB'd 5* solar memory more accessible, then the damage will even go higher.

Here are examples of a player who has a fully dedicated solar setup at the previous Bergamot Valley Nushi and Laeva.
https://twitter.com/aymskkt/status/1184052172943544320
https://twitter.com/aymskkt/status/1171240364159619072