Good practice Grant.
Back up mapping should always be kept separate, and I personally like to be able to see an entire sheet, as it gives you a much better understanding of the terrain, so like you always carry the OS sheet(s) for the area I am travelling in.
In SAR & Military I believe they are an essential carry because:
SAR - by looking at the entire area you may be able to determine that, rather than physically walking an area to search it, you could just as easily do this from a higher vantage point, with as good pair of binoculars; a mistake many teams make is to miss this fact.
Military – routes change and may take you out with your designated area. Plus, the reverse of their use in SAR for searching, you forward plan your route taking account of areas where your movement can be observed.
My only suggestion would be to consider printing your maps on waterproof paper that you can fold up and keep in your pocket, such as Memory Maps Tough Print Paper (although I think the price they charge is a rip-off: the cheapest I have managed to buy is 54p/A4 sheet) the product is good as it is completely waterproof and tear resistant.