Is their some particular reason why having inputs that can extend beyond the rails is so desirable? You are still exceeding the CM input range so the output will not be the representation of the input you intended, it will just clip.
Yes. If the inputs exceed the specified maximums, the opamp IC can latch that usually means it self destructs. So if your opamp circuit does connect to an externally power circuit that under some circumstances (eg power up) can drag an input more then 0.3V above the positive rail or 0.3V below the negative rail, then you do have to add a series resistor and a pair of external schottky diodes. You cannot rely on the inherent IC diode junctions as it it those junctions that cause the latchup in the first place.
What this means is that if you use conventional opamps in a circuit with multiple supply rails, and assume the supplies power up at different rates, you have to make sure that on power up, an opamp input can never go above or below the supply rails. With a LM324, you do not have to worry about this for positive overvoltages. With the LT1491, a 1K series resistor with the input means it is safe for positive and negative overvoltages.
As another example, I used LM324s in my power supply project. This means that I can have a 12V battery on the output, and switch off the power supply, and the LM324's can cope with the 12V from the battery fine without loading the battery. With almost all other opamps, I would have to add a series resistor, and the schottky diodes, and they would load the battery. In many supply designs, they get around this problem by using a reverse-biased diode that goes from the load output back to the supplies power rail which means that if the supply is off, and you attach a 12V battery, the battery is actually powering the power supply.
It is a nice feature. If you do not have overvoltage, you just have to make different design choices. That is all.
In the context of this thread, if you want to replace a LM324 in an existing design, you have to keep in mind that the designer of the original design may be using the LM324's overvoltage capability. If the design does not use this property, then replacement is probably not hard.
Richard.