I think there is literally no good point against implementing ActivityPub other than having no power to implement that (but we could start a fundraising to fund the development of AP implementation). There would be like 5 times more users to interact with.
One of the biggest problems when I try to encourage people to try Mastodon or some similar platform is that I hear „but there were many platforms like this and all of them failed”. If I would tell them that they would be able to communicate with Diaspora users from a new platform, that would make it pointless and make them feel like Diaspora actually didn’t fail.
I think finding problems in the fact ActivityPub is not so strict/doesn’t have a strict standard makes no point. There is a de facto standard but Diaspora could take for example Friendica as a reference for AP implementation, as I think these projects on pair with functionality. Mastodon is getting circles (think G+ circles, like Diaspora aspects) soon, so it would not be a UX disaster to combine both of the worlds.
And it would also definitely take shorter to write a proper ActivityPub implementation on Friendica than to explain every time that Diaspora does’t support AP 
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