I often hear, "You should only pick your wedding photographer if they use a particular brand of camera/lens." (Usually Nikon or Canon.)
In March 2014, I was introduced to the Olympus OM-D-EM micro four thirds (M4/3rds) camera system, smaller and lighter than my Canon kit but with equally good image quality when using professional lenses from the likes of Olympus, Sigma and Panasonic.
According to Olympus, the EM1 model at the time was designed to out perform the DSLR (Digital Single Lens Reflex camera) and is splash proof, dust proof but unlike the Canon system I was using, freeze proof down to minus 10!
I dipped my toe into the system but kept my Canon kit at the same time. However, in November 2016, I finally took the plunge and now exclusively use the micro four thirds system. For weddings, I use one Olympus OM-D-EM1 MkII with a 12-40mm pro lens and an EM1 MKI with a 40-150mm pro lens. I have a third EM1 MK1 with an Olympus 17mm and an Olympus 75mm lens.
The Pro lenses have a 35mm (think of film days) equivalent of 24-80mm and 80-300mm and both are dust proof, splash proof and freeze proof down to -10! So if I'm stuck at the back of the church or other venue as I have in the past, I can still capture those important shots such as exchanging the rings and the first kiss.
The other two lenses have a 35mm equivalent of 34mm and 150mm and are ideal for those full length/head & shoulder portrait shots.
With the three bodies and two Pro lenses, I can always carry on shooting if you're having a winter wedding and it's freezing outside. Your friend or friend of a friend with the cheap camera/lens will be running for cover by this stage.
So if my images have a "blurry background" this is done entirely for artistic reasons such as the image of the bride and flower shot and the groom in the mirror. The same goes if I cut heads off, this is done on purpose to emphasise a particular detail such as the bouquet shot below.
Ask your friend or friend of a friend what they'd do in the event of equipment failure!!. This actually happened to me at a recent wedding but I was fortunate enough to have the backup and it was quickly rectified.
I also offer the services of a second photographer as an extra backup and to cover your wedding from different perspectives, something your friend or friend of a friend wouldn't even consider.