This is just wrong. If we simplify the scenario and say that if the stealth rolls an evade, it cancels a hit (compared to say... being attacked with 1 hit, vs your 2 evades... the second evade from the stealth did jack as it was fully cancelled before the stealth came into play)
I wouldn't say it did Jack in that case - for one, it's presence manipulated the way your opponent attacked. He/She might have sent his biggest ship against your Stealth Device'd ship - in hopes of removing the device... and therefore sparing another ship from the threat. It's more than JUST a numbers game, it's also a mental game - though I DO stand by the numbers as the simple fact that it's only 3/4th the price of the SU speaks volumes.
The very idea that you can count on having a focus simply doesn't work, you can't count on it every turn for use in defense... Unless you're willing to forgo a kill in order to save that focus for defense. Not to mention the possibility of the other guy blocking your ship so you don't get an action in the first place.
I understand your reasoning, but I am looking at this from a natural play perspective. Typically, the first time I am attacked is likely going to be from a distance, range 3 or maybe 2. If my Stealth Device is still equipped by the time we are close enough to be bumping into one another... then as far as I'm concerned it's ABSOLUTELY done it's job.