I think what benefits those who approve of or at least tolerate futanari/dickgirls is that it's already such a niche idea that people who are into it, are generally accepting of a futanari character regardless of their personality, depth, etc. If I had to guess, the extra appendage that is the penis in addition to the overall feminine features is the main appeal. I don't see why some people would be so into futanari if they were not willing to receive anal or fellate a pretty girl's giant penis, otherwise what is the point?
What it comes down to is that people who like futanari/dickgirl characters will have much less to criticize when their fantasy or interest is being anally rammed by a pretty girl packing something that is equivalent in length to your own arm. Or bigger, sometimes. You always know what can be expected from a futanari character and I think it'd be a challenge to find futanari who are not interested in sexing you or who do not have that as an option.
Those female characters without penises? Not so much. When you examine them, they are not always intrinsically interesting depending on what the player is looking for or hoping from those female NPCs. There's a lot more potential for disappointment on a more individual level when they don't encompass an entire fetish or fantasy, unlike what futanari does. Some unmodified female NPCs, as in those without a dick, may be written as too dominant. Some may be written as too submissive. Some are just there to be a throwaway. Some may not even want to sex you at all because your own character's penis is too small or you aren't the correct gender. I know this was something we could experience in TiTs, although I can't remember if it's the same for CoC 2.
People like to refer to the statistics to disprove the opinion of "too much dicks in my porn game" where the Wiki states there's over 100+ unmodified females in comparison to 30-something futanari/dickgirls . . . but the statistics don't take into consideration just how many of those females are genuinely quality over sheer quantity. It fails to detail just how many of those female characters will properly satisfy certain fetishes players, as individuals have, such as how many of them are more dominant or submissive, or whatever it might be. So, players are spending a lot of time running around searching for those 100+ female NPCs, trying to locate the ones which will arouse them the most based off of what they offer. There could be 100+ female NPCs but when you consider how it's basically quantity over quality, there's more like 40 female NPCs which are actually decent enough sexually, making it more of a tie. Hence the complaining about too many dicks.
I think people who like or prefer futanari are more likely to be satisfied because it's generally the same idea each time.
