Akismet is a paid service and their apis are tailored for comments. An advantage with comments is that you can just mark as spam if some contents have dubious links or keywords.
An issue you have in many forms (e.g.: login form) is that there is limited data to decide if it's a real user or a bot.
Agreed on all points. That's why I said in my original comment: "I know these get used for more than [contact] form submissions, to stop bot sign ups etc…"
I picked up Akismet because it's been around forever, and while it is paid, it is very cheap for my use case.
This is a bit of an aside, but I feel like Automattic is sitting on several companies/products and not doing a whole lot with them.
Akismet could be expanded into a more fully featured server side spam detection SAAS with a flexible API etc...
Gravatar could be expanded into something like OpenID.
An issue you have in many forms (e.g.: login form) is that there is limited data to decide if it's a real user or a bot.