All manufacturers will advertise the best numbers to make the product look better than it may really be.

Nikon state that in the specs that the viewfinder coverage is 95% both vertical and horizontal (D5200). Overall tho this adds up to a tick over 90% for the entire frame.

95% sounds a lot more marketable than 90%(total area) .... which it really is.

For some this may not be an issue, and of the few fellow Nikon owners I know with these types of cameras, they've never complained about it.

But I tended to do so with my D70s, which was also this supposed 95%. As I was predominantly a landscape chasing type, one of my main priorities was to frame the scene without the need to crop, at all ... not even the annoying millimeters or so that I used to get with the D300 as well on the horizontal axis.
So even the D300 only has 99.9% coverage according to the viewfinder, which when compared to Liveview would sometimes leave a small obstruction on one of the sides(Landscape orientation) that was not seen via the viewfinder.

So far the D800 is showing true 100% in both vertical and horizontal coverage. Haven't actually looked for any discrepancy tho.

A D5200(to me) would only ever be a fun camera to use. Too many limitations compared to what I like to have at my fingertips.
Of course I've never actually used one, my closest experience with a D5200 has been with the D5100 in terms of body type and a D3200 in terms of 24Mp sensor.

Nikon's timing has to make you wonder tho. Why'd they release this camera with this sensor so far down the track given that the sensor is already available in lower end cameras.
Surely they must have tweaked it considerably more than a simple omission of the AA filter given that it's now more than a 1 year old design!
And now that their marketing strategy has taken a turn, of course there is every chance that a higher end D300s replacement is still in the works too.
Nikon don't usually reveal the origins of their sensors, so there is every possibility that it may be different in specification to the D5200 sensor.