Re: Low magic is fine but why this?
The reason that I think a casting roll to cast a spell is useful in a low magic setting is because of this: a setting would really only be low magic if magic were in some way difficult/dangerous to deal with. Its fine to use the regular D&D spellcasting rules in a low-magic setting, but that leaves open one large problem in logic- if magic is sure-fire and predictable every time, why don't more people use it? One way is to strongly restrict access to magic, but then you are stuck with the same mechanics and rather dull spell desriptions in the PHB. The other alternative is to make magic less predictable and harder to control, which opens up whole new roleplaying possibilities.
Example: A witch (someone who communes with nature spirits) in my game tried to cast a spell we made up called Contact Spirit World, which basically allows the witch's spirit to roam a limited area in the spirit world while her body remains in a trance. The witch in question botched the roll badly, and instead ended up sending her body to the spirit world, while her spirit remained in the physical world. Knowing that if something wasn't done quickly her body could be inhabited by a wandering spirit on the other side, the witch "haunted" the other characters until they came to the place she had performed the ritual, and the priest cast Dispel Magic to end her spell. I can't even begin to count the number of times, some small (or major) effect like this has happened in the game, but from speaking with my players, I know they all enjoyed it immensely, and they enjoy the unpredictability and role-playing through the possibilities. I know this type of thing may not be for everyone, but if you are using a low-magic world, there has to be a reason magic is difficult to work with.
I do agree that increasing the casting times of spells drastically is generally not a good idea. If you are requiring a casting roll as well as increasing the casting time, many spellcasters become burdens to a group unless they have a lot of time to prepare. More importantly, the player of such a character feels like he can't accomplish anything, and player enjoyment is the most important thing.
Also, I have noticed a reluctance on the part of several posters to use a spellcasing roll to determine if a spell works. I have played probably 30-40 game systems over the years (including Chivalry and Sorcery, Ars Magica, Rolemaster, Runequest, Pendragon, GURPS, Deadlands, etc), and to my knowledge, D&D is the ONLY game system I know of where magic works without some kind of spellcasting roll (barring wearing armor or disrupting concentration). To me, this only adds to the video game feel of D&D, and relegates magic to the level of a technological device, which is often boring. While this mechanic works well in a high-magic world, for a low magic world that is supposed to be darker, it takes away from the suspension of disbelief.
As for why anyone would be a magic-user in a world like this- well, it isn't for everybody. If a player wants to play the mysterious stranger who has superhuman powers, I let him know magic powers don't always work, and the road to arcane knowledge is a difficult one. However, most players in a low-magic setting realize that acting strange because they are magic-users is about the best way to get normal people to pay attention to you, and if we are assuming that in most low-magic worlds people are afraid of magic, this is about the dumbest thing a wizard can do unless he has a LOT of power to back himself up. While it is true that in a low magic world a wizard's spells don't always work, when they do work, they can do wonderous things the normal person could not dream of (view a distant place, bend the will of someone to your desires, call down a curse, etc). And because magic is less common, if the wizard is subtle, people are not as likely to believe that a spell has been cast as they are to attribute the occurance to chance, or some otherworldly power that is not the wizard. To be honest, in a low-magic world, a spellcaster has to be played a lot smarter than in a typical D&D game- magic is no longer fire-and-forget, but something the aspiring wizard struggles to control.
Finally, I have heard many people say that if we don't like the high-magic nature of D&D, why do we use these rules? Simple- although there might be better systems for handling magic, D&D has a huge fanbase and reletively simple mechanics to keep track of, and this means its easier to find players, and not as large of a workload on the DM to run adventures (as compared to something like...Rolemaster). Tweaking the D20 system to handle our tastes is a more viable option than trying to teach people to play a whole new game system, and in most cases the changes that are made don't complicate the game any more, but from experience, I can say they DO add greatly to the atmosphere of the game. Sorry for the book, but I hope this helped to clear up my reasoning, and can maybe prompt more discussion.