What is your opinion about the theory of transfer?
In general, the theory of transfer makes sense. However, it may be overstated in that it seems likely that there are times when any transfer would be minimal or would be so easily obscured that it would be of no benefit to investigators.
It is surely true that any person or object that comes into contact with another leaves some sort of evidence behind. If I brush against a wall as I walk down the hall, cells or even hairs from my arm might brush off onto the wall. Of course, with more significant contacts (me punching someone or a car hitting someone) there will be more transfer.
However, this does not always mean that the theory of transfer will be helpful to an investigator. If I walked down that hall and left only skin particles, it may well be impossible for investigators to ever find the evidence that transferred from me to the crime scene. So, all forms of contact leave evidence as transfer theory says. But that evidence might be so minute and impermanent (if the skin cells blew away, for example) that it might be of no practical use.