I'm still waiting to receive my ambulance, but I find the mixed reception to it very interesting.
On one hand, the GT series always had its fair share of oddball cars. I'd even argue that they are an integral part of the soul of GT. I'm looking forward to trying out the ambulance for the first time, but I doubt I'll drive it for more than 10km before it goes back into the garage to collect dust, not unlike an Aston Martin Valkyrie or an AMG GT3 '20.
While oddball cars have always had a place in GT, I think that as the games strive more and more towards realism, they've lost the feel of a fun game to play. The vast majority of in–game races task the players to enter an obscenely overpowered car to overcome a 45 second deficit to the race leader within 5 laps or so, which pigeon holes players into choosing the select few "good" cars to win. Hitting the unpredictable AI cars loses a third of the race winnings, and hitting walls results in a 5 second penalty on top of the slowdown from hitting said wall, all while incurring damage. It deeply deincentivises fooling around and experimentation, and demands perfection from its players. Such a gameplay style is at a total tonal clash with having a playable ambulance, and that's even before we consider the fact that it's only eligible for one–make races in custom races, and gosh knows how many World Circuits events it can actually enter, let alone be viable in.
Back in the earlier GT titles, races were a closely contested standing start, and skilled players could choose off kilter picks to challenge themselves. Mistakes like hitting the AI cars or walls weren't punished at all. There was less consequence for suboptimal play, and therefore there was less funneling players into a competitive car. There was more room to experiment, play, and fool around. Winning a late–game race populated by high powered tuning monsters or bona fide racecars with a car that one could see on the street frequently was a trophy worth more than anything the developers could've ever given a player.
Unfortunately, I seriously doubt this is possible with an ambulance.
This problem's extent doesn't just stop at oddball cars; it also encompasses the racecars people clamour for to be added in updates. As we've seen with the Skyline Super Silhouette and GR010, cars that aren't the quickest in Time Attack leaderboards are immediately ignored and forgotten, not unlike a HiMedic. Brands with multiple cars to represent them in Gr.3, such as Toyota and BMW, all have a single, undisputed fastest car, instead of cars with balanced strengths and weaknesses. In previous Gran Turismo games, winning a Castrol Supra was a big, joyous occasion that opened up so many different races with bigger rewards in the campaign. Seeing a Castrol Supra in my garage in GT7 makes me think, "eww, slow." Having an Escudo made me feel like an invincible king in GT2; having an Escudo in GT7 just makes me think, "urgh, I can't do ANYTHING with this!"
I don't have much time in a day to play GT7, if at all, and when I do get the chance to sit down in my rig, half an hour of playtime goes into one of the "big four" grind events, simply because cars are so expensive in GT7. And when I play a grind race, guess what kind of car I want for the chore. I'd obviously want the fastest car that can get the event over and done with the quickest.
Ultimately, any car that gets added into the game feels completely inconsequential unless it happens to be a personal favourite, because the game doesn't incentivise the use of a good 90% of its car list. I think the root cause of the "what am I supposed to do with X car?" and "Who asked for Y car?" comments stem from the game simply not being a very fun game.
Hope all of these makes it to GT in the future (along with 296 GT3, Conti GT3, 720S GT3, 992 GT3 - see I'm not biased 
