As with fake bokeh, it is easy to fake a motion blur in uncomplicated cases. Where the blur direction is uniform and running perpendicular to the focus plane shouldn't be too much of an issue. In many cases where you'd want motion blur, this will be the situation.
That's exactly true, which is why it works just fine without much fuss at all in the majority of cases.
But, say a runner or cyclist moving at an angle to or from the camera, getting motion blur that is physically accurate won't be easy.
It doesn't have to be "physically accurate" in the strictest sense of the phrase though, and we should keep that in mind; It only needs to be "acceptably believable". As long as it doesn't call the wrong kind of attention to the viewer, it's fine.
The techniques to manipulate and control perspective, direction, opacity, curvature, etc aren't a mystery, even in the cyclist situation you describe. Obviously, it's not as easy or straightforward as the straight pass mentioned previously, but it's not that much more complicated if one is familiar with the basic tools and techniques in PS.
Speaking of control, in some cases super-smooth very controlled blurs that are impossibly perfect (think of comic book hero The Flash's trails) might be exactly what's called for in a particular piece, rather than a real blur as-shot that doesn't provide that kind of effect, and certainly not the control one might desire to have over the rendition of the piece. It's just another reason to have that skill in the toolbox, even if seldom needed or used, IMHO. And again, it's just knowledge of the tools and skills used throughout the program, applied to this.
It just depends on the level of complexity and realism that you're going to need.
Which is, again, "acceptably believable". No more, no less.
A bad fake blur though will always be distracting.
Well, yeah... a bad anything will always be distracting, fake or not. Bad light, bad pose, bad expression, bad white balance, bad background, bad foreground, bad (_______ fill in the blank another 10,000 times).
So yeah, don't make bad fake blurs, everyone. In fact, don't make bad "real" blurs either.
But IF you find yourself in a situation that calls out to you to make a fake blur after the fact, then definitely make "acceptably believable" fake blurs, not "bad" ones, because, let's face it, "bad" is not "good".
If you don't know how to make acceptably believable fake blurs yet, but would like to be able to, then take some courses. They're not terribly expensive, and it's well worth having those skills - for whenever you'd like to use them. It's not rocket science or brain surgery either. With a bit of knowledge, practice and patience, anyone can learn to do nearly anything that can be done in Photoshop, short of digitally painting a masterpiece from scratch on a blank canvas, or something like that.
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