If your child doesn't need extra time I don't know what the problem is really.
Our teen just sat GCSEs and I asked him if he would have benefitted from extra time (he doesn't need it was just a part of a general "how was the exam" chat) - "god no" was the general answer, and this included Eng Lit!
He said he would second guess his answers, probably go back and change stuff detrimentally. If you get the full 25% in Eng Lit it's over 3 hours in an exam, that would not help at all. Apart from the English exams where he was writing until almost the end, he didn't take any other essay subjects, but finished in time, with time to check over the paper, he was happy to be done and out in the time allocated.
It's not an advantage if your child doesn't need it.
So why people think it is. If you have secondary school children sitting exams, talk to them, they will probably tell you the amount of time in an exam is fine, if you don't know something, thinking about it for an extra 25% of time isn't going to make you suddenly know it.
If you break it down and work it out between all the questions it works out at seconds per question extra, it's not a lot.
It's not an advantage if you have an able child, so if you do have a child who doesn't need extra time there really isn't any need to feel it is unfair if some do for valid reasons.