The problem with electronic transactions is that they leave a trail which can never be deleted. Unless you are using something like Bitcoin both parties will have eachother's real names and bank account details which is no good.
If a wg falls foul of the taxman and you are transferring money to and from her account electronically you can bet that your bank account will come under some scrutiny. Furthermore, if something like the Swedish Model comes to pass where purchase is criminalized for punters having financial transactions between yourself and a known wg would certainly be evidence to implicate you. Cash is king for now.