What is the point to beat Gengar in raids
Gengar's best moveset is now legacy, and the TM ratio for tier 3 raids is quite low compared to tier 4. Besides, a Focus Blast Gengar can demolish your Tyranitar and several Hexes can kill your Gengar. In my opinion, doing Gengar raids is wasting your revives and (premium) raid passes, you get less benefits compared to other raids.
Answers
People do lower level raids when they don't have enough people...
Like wife and me can do any level 3 for the best bonuses possible for 2 people (assuming you aren't level 40 with fast phones and can duo Tyranitar)...
That said, I never use a "premium" pass on a level 3 raid because if you have enough people, always do Level 5 and get more stuff.
I've done 6 tier 5 raids, not an extreme amount b/c they are everywhere. However, In that time I also got enough xp to go from level 1 to level 27. I think a lot of low players really lost an opportunity by trying so many tier 5 raids and not getting xp and hatching. Also, Gengar's moves have changed before and they will change again.
What is the point to playing this game? To have fun! What is the point to beating a Gengar Raid Boss? To have fun! The most fun I am having in this game at this moment in time is soloing the Tier 3 Raid Bosses. I got Arcanine today. I think I have now built the teams to solo Machamp, Gengar and Vaporean. When I find them close to my house, I will go get them. It is all about having fun. Just like I went fishing last weekend and hooked a 150 pound Mako shark (released unharmed). That was all about fun. Virtual or real, catching creatures is fun! We humans have been doing that our entire existence.