This was originally an episode that I figured you could skip if you were so inclined, with nothing but fairly generic material contained. On the one hand, it could have been rated as something fans of the franchise might watch, as it contains thematic elements featured in various incarnations of Star Trek, including non-corporeal beings and crew possession. If that's your bag then you might still consider that a reason to watch.
Yet "The Crossing" might be considered equally for partisans of Trip Tucker, who's the nominal lead character of the episode, even though most of his scenes are under the influence and control of the wisp beings who flit about the ship and spend moments with several other characters. It's a fine performance among many fine performances from Connor Trinneer (I don't reference actors by name too often in these recaps, so that may mean something), already at this point in the season well on his way to establishing him the most important if not simply breakout star of the series (although his career since Enterprise has been pretty nonexistent, which is horrible). It's a rare chance for Trip to be on the other side of the mischief he's more usually instigating himself, as different from "Unexpected" (in which he famously gets pregnant) as you can get.
But you can also watch it simply as an ensemble piece, something that's always fun with these characters. It's fun with any cast, but these ones in particular are good at maintaining a sense of continuity and purpose even when the situation doesn't really go anywhere as far as character growth goes. If Star Trek had ever considered simply hanging out with a set of characters, this would have been the cast to do it with.
There you have it, then, three different reasons to care about an episode that might've been skipped under a different set of criteria. It's not earth-shattering, but it's worth a look.
franchise * series * essential * character
Memory Alpha summary.