Pun intented here.
If you do a lot of pair programming think again
here are some reasons why you should use your own keyboard at all times:)
1) Your pair has a virus ( real kind ) and is sneezing all over the keyboard
2) Your pair had a flu and is recovering from it.
3) Your pair is holding fries in one hand as he./she types and uses the keyboard with the same hand . Umm french fried keyboard.
4) You turn your pairs keyboard on the side and see all the food particles and dandruff fall.
5) Your pair went to the bathroom * enough said:)
If i have convinced you enough carry your own keyboard for pairing sessions and use purell when in doubt:)