EN gives you save throws against critical effects, probably all critical effects except for bypass (it's LK based), but I'm not sure. It means that you'll be less likely to knocked down/out, crippled, instakilled.
And then you can take Stonewall perk with 8 EN only if you're planning to eat buffout and can't, for some reason, have 10 EN to use that perk on maximum.
But really yes, EN from buffout and psycho in general almost useless and of course it should change your HP in real time, so even 9 7 5 3 EN builds could be viable if used with Psycho +some drugs can negate EN. But it belongs to suggestions.