If you don't have high end speakers, another pair for reference is very handy indeed. Actually it can always be handy. Low-budget speakers often have problems in certain ranges and in transparency overall, which is exactly why another set could help you spot these problems. But IF you have good monitors (about 2000E+ budget) & acoustics to start with, a reference set should not be necessary for a good result . I use productions which have proven to be solid on lots of systems as my reference. Which imo should be enough of material to refer to on just one speaker set .
On those B&W speakers you can here a needle drop on the floor even if it was on a 100m distance from the mic. They're insanely accurate and amazingly transparent (given you got the separated amps of about 9000Euro per speaker). If your mix sounds awesome clean on those speakers it sounds awesome anywhere. 100% guaranteed. There's a reason why a set these speakers (with amps) cost about E40.000,- .
I recommend you to listen to them in your dedicated speaker shop. You will immediately get depressed about any other speaker you hear. My life will never be the same. And if I had the money to spend I would most definetely spend my 40K on a set of B&W's. It would be my most precious possesion and I would treat it better than my lady.