Yeah, I'm also not at all inclined to arrange a fight between Marisa/Reimu and the spectre so that we can intervene with force to save her. Even in Marisa's case, we'd essentially be trying to unleash someone against her who would intend very serious harm. Marisa's been a hard-headed jerk for most of this quest, but even that doesn't mean she deserves to have her shadow stolen or be shadow-swallowed, and I would expect the spectre to try one or the other, if she could. And if Marisa is occupied with us, there's even a chance she could succeed. There's a good chance she doesn't even know the spectre's capabilities, unless Reimu has spoken with her since meeting Tenshi, making her all the more vulnerable. Sure, we may also be able to intervene to stop this, but that may potentially require using force against the spectre herself, more than erasing any potential gains.
Essentially, even if we didn't care about being a jerk to set the thing up in the first place, there's plenty of things that could make it end quite badly.
I also wonder how much good it would accomplish, even if it was executed perfectly. Certainly, we could point out that she would have lost if not for our help. She may or may not believe that (she's certainly been plenty confident about her abilities, even when not justified). And even if she does accept that, and is even grateful (after a fashion), it's a reasonable argument that assisting against a mutual enemy isn't really exposing oneself. At least not any more than drinking with Suika was. Neither party needed to tell the other anything, or share anything, other than each others physical presence for a short time.
In terms of other options, I've been pondering the 'need' concept for a while now, and I'm afraid I'm still drawing a blank. My gut says we're not suppose to interpret it in the most obvious way, but this is rather useless in the absence of any actual ideas of what other way we're supposed to be interpreting it. I wish I had something more useful to say.