I don't know about Leon, and I agree about Tengu. But people actually DID use Ein, high level and casual players alike. He was used in DOA2 and 4. His omission is legit, not on par with SCV's character omission but he was certainly cared about and hardly considered a "nobody".
And it is legit to complain about characters regardless, even ones you don't use. Because cutting characters from fighting games, especially fighting games is detrimental to the game and to do it for story purposes? You can't really compare that to Tekken, Asuka more or less WAS Jun with a different model and attitude. People just actually liked Jun. Ein and Leon on the other hand have notable differences from their counterparts and replacements (Hitomi), enough differences to make a matter in competitive play, which is kind of the point of fighting games to begin with.
You are right however, they don't make or break the game, but it would be a better game for many, if they were in it. Not even going to comment on SCV anymore, literally cut every character I used and liked from the series, I gave up at that point. I'm done with SC.