>Pullup resistors have two common uses.
>
> 1.  They are used to pull unused input pins to a Logic High, usually
> five volts. While you can tie inputs directly to five volts, a resistor
> is often used so that at a later time an output from another device can
> be connected to this unused input without harm.  If the input is
> connected
> directly to five volts, then this direct connection would have to be
> removed before an output from another device can be hooked to the input
> pin.  With a pullup resistor, the connection can be made without
> removing the resistor.  This is because the resistor limits the sink
> current into the output of the driving device to a safe level.
>
> snip...

        Historically, the pull-up resistor was required with unused
        _TTL_ inputs to limit the current into the gate (and this is
        where the later reference to a 4700 ohm resistor derives).

        This had nothing to do with subsequently wishing to connect
        a signal to the unused input.

        The nature of _CMOS_ inputs is such that a resistor is not
        needed to limit the current.  A CMOS input can be connected
        directly to a power rail without excessive current drain.

        In my experience, it is not standard practice to include
        pull-ups on unused CMOS inputs.

        The argument given for using pull-ups is valid.  Its level
        of persuasion depends on the particular application.

___Bob