I was kinda expecting the trope where the holy sword can't be unsheathed by anyone but the hero, and it was a trick by Alexis to try to get him to expose himself as a special existence, or something.
No, that's the wording for if he said "if the item's true function were discovered, it'd be harder to be exiled later", not "to avoid being exiled later". As written:
"If this thing's true function were discovered, it would make it much harder to avoid being exiled later"
is
"If [I'm too overpowered/too useful], it would make it much harder to [stay / not get exiled]"
It's not a question of him being useful ("why would the hero party kick someone for being competent?"), he's being worried about being too overpowered, and arousing suspicion, just like in the start of this chapter / the end of last one. Alexis is very clear about how there shouldn't be any other humans able to match his abilities, which means this "friendly ally" coming into the party could be a demon spy or something.