To be honest, it depends what exams are seen as hoping to measure. I used to think that people shouldn't have extra time in exams; people don't get extra time in life, so to speak, so why in exams? The whole point in exams is to measure mental abilities, to be applied in 'real-life' situations, after all. If exams are set to measure things which some people struggle with, then clearly it is absurd to give extra time to these people. However, if the exam aims to measure something else, and, say, the facts that questions have to be read, and answers given in writing, are purely irrelevant to the assessment, then it makes sense to award extra time. To be honest, though, giving people extra time in English exams seems ridiculous; the whole point of which is surely to measure one's reading and writing skills, so to give people extra time for............not having very good reading and writing skills, defeats the purpose of the object; you might as well just give them extra marks, automatically. However, for subjects such as Mathematics and Science, it is more about reasoning and memory, and, hence, not so much about reading and writing, so it is perhaps unfair for students to be hindered greatly by a lack of verbal skills, when it comes to these subject areas.
In my opinion, it all depends as to what the situation will be, when outside of the classroom; reading and responding accordingly in writing, is going to be important, whatever the area of study. This may not be strictly related to any given field, but it will still be essential; if some people struggle with this, then, well, they struggle with this, and may not achieve quite as much as those who don't; and exam results should reflect this, for potential achievement in later life is surely what they aim to measure.
So, I suppose I'm undecided, on the matter, but do feel that extra time is awarded too easily, and too generously. Besides, people vary in the speed at which they read etc, anyway; you can't account for everybody's abilities. I also think that the 'dyslexic' label is awarded too easily; with all respect, when I was at school, most of these so-called dyslexics were just stupid. I'm not doubting that the condition exists, affecting people regardless of intelligence level, but do think it is open to some considerable misdiagnosis, to be used as an excuse for those who generally just aren't very bright, and are, accordingly, crap at reading. Find any idiot, and you can bet your last dollar that s/he's 'dyslexic'. And these people, effectively, will be being awarded extra time in exams, for stupidity.
As Rory McGrath said: "You'll never hear anyone say 'my kid's not dyslexic; he's just stupid'".