I'm not f2p (I buy the battlepass, buy the monthly crystal pass, buy costumes I want, I'm what it's usually called a dolphin) so I have more wiggle room compared to others, but for me there are enough benefits to get all new dolls compared to waiting (but I never rolled for dupes so far, and probably never will). I think with a bit of luck (like not always losing the 50/50) even a f2p can get most dolls (at most skipping 1 every few months)
First, the most important element is "getting the doll". By getting a doll you can now level her, limit break her, unlock her neural helix, all things that are useful regardless of the rarity or the amount of dupes. Even the highest tier unit needs to be level 60 and fully set to be able to use her effectively. By having every doll I know I have the choice to "grow" anyone I need if necessary
Second, getting more dolls means your dispatch room can be levelled further. The dispatch buffs can be good and are applied universally to each doll of a certain class (even a new level 1 doll has them). The Dispatch bonuses are probably the easier way for f2p to get stronger unit without rolling everyone
Third, eventually the game requires more than one team, and a team itself can change a lot especially if you are looking at elemental affinities. Dushevnaya for example is the 3rd (well 4th with Lotta but she is SR) Ice element SSR. Her skills buff Makiatto's damage even further, and together with Suomi she can apply more freeze debuffs that further boost Makiatto's damage. You don't need Dush to make Makiatto's high tier, but making a full ice team makes it much easier to deal with the hardest bosses faster (like in Boss fight mode, or in Gunsmoke Frontline). By having all dolls you are ready for any team composition (I always try to keep 18 cores and enough cylinders to get a doll to max if needed)
Fourth, but this is a personal one: modern gachas are putting a bigger focus in making a character "worth" even if they're low tier. The dispatch buff is a perfect example, but you also have things like affinity systems, well modelled graphics, costumes, specific voicelines, exclusive stages or unlocks. Basically all things that are unrelated to the actual gameplay, so features that benefits top tiers and shitters alike
But the biggest part is that I only play one gacha, and one alone. Well all right I also play GF1 and its spinoff Neural Cloud, but I was fully f2p in GF1 for the entire run, and the game is almost at maintenance mode in global. Same for NC although I did spend something with that (and that one is also on its way to maintenance mode). What I mean is that I spend on GF2 the equivalent amount of money that I would spend on a normal game with a similar quality. If I got enjoyment out of GF2 and I end up having access to all content (so the dolls) without wasting hundreds of € then so be it. If one day the game becomes too greedy I will most likely stop all spending and rolling and just ride it out until the end
