I understand where this request is coming from - I've spent hours removing spam wave posts on sites that are struggling and it would have been convenient to have access to flag without opening the question first. Even still, I don't think I would have wanted this feature added.
My primary concern with this request is that it only allows spam flags from the homepage (with the possible addition of mod attention) - which means that there is a likelihood that people might start flagging things incorrectly - for example, spam flagging questions that are clearly off topic - but not spam - as spam to save themselves a click through to the question itself.
Of course, no one should do this - that doesn't mean they won't. Particularly considering this request doesn't include any limitations about which users have this option. With only 15 reputation required for the privilege, far more users have the ability to flag posts than many other actions. When you give people an icon to flag, they will inevitably use it - often incorrectly.
In reviewing flags on various sites over the years, I've found people would frequently select the incorrect reasons for flags - or flag posts that shouldn't have been flagged at all. And, while most flags can be cast incorrectly without any impact on the poster, spam flags automatically downvote the post, which can be harmful in cases where the flag is unwarranted.
That said, most flag reasons can't be determined from the home page, so expanding the menu much beyond spam, abusive, and maybe off topic closure doesn't make much sense.
And this is all in addition to users not being able to see the full body of the post to be certain it's truly spam. Maybe if the flag UI in this situation had a post preview to encourage confirmation of the flag reason - which would also allow an expansion of the options available - that would assuage concerns more but that becomes a much bigger change and doesn't differ much from the status-quo.
For that reason, while I understand the request, I don't think it should be implemented as described for all users on the platform - or even limited to users with the association bonus. Instead, for users particularly active in flagging spam, I'd recommend using userscripts to add such options, assuming that's possible. Perhaps an expansion of the FIRE userscript that Charcoal has that works on question lists instead of in chat?
Additionally, if a site is experiencing a huge spam wave - particularly one lasting multiple days - there are settings that CMs can adjust that may reduce the volume of spam.
