When it comes right down to it, your alkalinity is a measure of unused buffering capacity. If you keep adding buffer after your pH hits the buffers capacity, than you are just adding alkalinity, carbonate hardness, whatever you want to call it (to a certain point...you can overbuffer, but it takes alot). It is pretty much harmless, but wasteful. I know people who claim they keep their alkalinity at like 25dKh, which is just moronic. It is like when I see people saying they have 550ppm of Calcium and I try and tell them, "no, you have an expired test kit for calcium".
Having this excess in the water means that when conditions change, such as at night, the tank just absorbs the swing and the parameter doesn't change.
When the lights go out, your plants and corals stop consuming C02 and producing 02, and start consuming O2 and generating CO2, much like our respiration. The effect of this is that the concentration of CO2 in the water increases. It interacts with water, and this causes more carbonic acid (H2CO3) to form, acidifying the water and thereby dropping the pH (this is also what frees up calcium in your tank by dissolving rocks and sand, which is why your pH will never get below 7.8 IME).
This excess buffer leaves a bunch of carbonates and maybe sulphates and whatnot in the water, and rather than floating around mucking everything up, those carbonates join up with the free acid molecules, neutralizing it and forming the likes of HCO3-, CO3 and other harmless molecules. There is a whole big chemical equation that balances out and is pretty cool, but I haven't been in school for too long, and you can easily look it up by googling "carbonate cycle" if you are into it.
I have never had anything die because of a pH issue, but I am always careful since it usually isn't my tank that I am working on. I would stick with bergy's "dissolve in ro/di or even tank water, and then add over the course of a day or two." When you do manage to fix it, you will find it requires much less tinkering than getting it right the first time, which can seem like it takes forever.
