The problem here is that if this circuit ended up entering one of the unused states for some reason you would be stuck in a loop then never returns to one of the used states. This counter has the following states:
00000000
10000000
11000000
11100000
11110000
...
00001111
00000111
00000011
00000001
If a cosmic ray hits your chip and flips one of the bits then you could end up with states that don't exist in normal operation:
00010000
10001000
11000100
11100010
11110001
01111000
10111100
....
The only way to get back to normal is by asserting reset. You have two options to deal with this: either add a bunch of logic to detect the situation, or assume that a bit randomly flipping isn't going to happen. I would say that you don't need to worry about it unless that flipped bit is going to launch a missile.