Yeah you are right the 3*/4* stuff is a bit missing the point. But if your luck is really bad (the legendary bronze pull exists ) it could be a long time until you get a full party of Gacha 5*/Event 5* and in that case it's convenient to speed up the process( i did it). My luck is not the worst but far from the best cause i always get 5* i am not a fan of...
There are even some guys quitting Nutaku FKG recently because of there bad luck and gem starving. If there luck will be better on DMM is another Story but they have more chances to roll something good cause DMM Players get much more free gems than us.
Your luck is solid though cause you already got 2 Rainbows and 5 Gacha Golds. You also started at the right time (Black Friday Deals, 5* Tickets etc.).
Let's say it's convenient to pay for Girls you actually want.
Not all Event Girls are good though. Heck there are a few Event Girls which are weaker than some 4*. Calla and Gentian are considered one of the weakest Event Girls. Same goes for Gacha 5*: Gerberea has lackluster Abilities and stats (HP is really bad, her attack stat is good), Bipinna Cosmos isn't that good until she gets her Skill Buff (damage becomes 1,8x to all enemies) and her Bloomed Form. She works decently in a Shine Crystal Collector Squad but there are still other Girls outclassing here.
So unless the mentioned Girls are considered waifus it's better to replace them asap.
And yes i agree: If you are a Collector and you want all available Girls it's definetely P2W.