Why is it important not to exceed Vcc at the input to a logic gate?
Almost every IC you can buy has a number of "hidden features" that are assumed to be present and thus not discussed in the datasheet.

Among these are body diodes/ESD suppression diodes. These guys generally hide on every I/O pin on every device, from basic logic gates through memory to high-end microprocessors. They route any voltage that is greater than VDD (supply voltage) or lower than VSS (supply common) to the appropriate rail.

If you apply a voltage in excess of either of these limits, the body diodes become forward-biased and effectively clamp the level at the pin to either VDD or VSS. This sounds like a good thing and generally is, but they are very small devices and cannot dissipate much power. You can end up damaging this diode (shorting it or blowing it open). In the former case it can lead to "stuck" I/O pins, and in the latter case, the next overvoltage can destroy the input.

Open-collector outputs are handy for being able to control some outputs, as pingswept mentioned already. Putting small resistors in series with inputs which may come into contact with nasty voltages, and/or using external diodes (even an 1N914 is HUGE compared to the protection diodes on the IC itself) is a good way to help protect devices.

Of course, properly designing your input or output circuitry to handle continuous or repeated transient events like these can be a design challenge in and of itself. Generally speaking, if you are worried about blowing an expensive part, buffer the input or output with (much) cheaper and preferably socketed buffer ICs.