Using an RFID reader on an Arduino that wants to be doing something else presents some issue. If the RFID reader is connected to the TX/RX pins, so that Serial.available() works, it interferes with uploading sketches to the Arduino.
If it's connected to other pins, using software serial means, the speed may not be fast enough to capture the data, and not all the software serial options implement the Serial.available function.
Touch screens and lcd can use up a bunch of the Arduino I/O pins, leaving few free for other purposes.
Why would you want to shut off the RFID reader?
Multi-tasking has many definitions. There is no threading model on the Arduino. That is, several processes can not be running simultaneously. However, even on a single core CPU on a computer, only one process at a time executes, regardless of whether an application is multi-threaded or not. Use of interrupts makes it possible to not continually ask a sensor or device if it has information of interest. It will butt in when it does.