Firstly, it is interesting to note that the 'attunement' methods used by Mikao Usui's Reiki Association in Japan (the Usui Reiki Ryoho Gakkai) do not involve the use of symbols: the Gakkai's students are connected to Reiki, but the Reiki symbols are not used in this process.
Secondly, Hiroshi Doi - a member of the 'Gakkai who has brought their basic teachings to the Western world - says that Mikao Usui introduced the symbols into his Reiki technique for the benefit of his students. The story was that some students could just not accept that they could 'do this thing', and by giving them something concrete to use, they learned to focus and control the energy more easily.
Hiroshi Doi also says that the current 'Gakkai students do not use the Reiki symbols, though they are shown them out of historical interest. Mikao Usui no doubt chose the symbols very carefully, to represent different aspects of the energy, but the energy and the connection to the energy came first it seems.
So perhaps the message we should be getting from the above is that we may choose to use the symbols if we want to or need to use them, but they are not necessarily an essential part of the system, and if we feel that we can move beyond them, that is ok, and that is still Reiki.
I have been in contact with a Moldovan Doctor (and Reiki Master) whose Reiki friends use Christian symbols to control and focus the Reiki energy; I also know a Moslem Reiki Master who uses Moslem prayers to do the same thing. From personal experience I can say that it is possible to produce the desired effect with the energy through intent only, whether that be distant healing or gearing the energy in the direction of mental/emotional balancing or physical healing.
For me there is a distinction to be made between the energy of Reiki, and any symbols that might be used to control and focus the energy in a particular way. Reiki is the energy, you are attuned to the energy, but there are many ways of directing the energy, by using symbols from Japan, from other cultures and through simple intent.
Taggart King, Reiki Evolution. www.reiki-evolution.co.uk