"On the spamming thing, it seems like there are a million and one ways to automate emails. What's one more? "
If there is a million and one ways to automate sending emails, any reason you're not using one of those?
"Who could complain about an email client automating emails?"
Just one possible scenario: someone who receives such spam message, with "Sent with AquaMail" in the signature (!!!) and a link to my web site (!!!!!) and then reports it to Spamhaus or whatever... From where it might make it to corporate mail filters, and who knows what else.
"What about wildcard (or RegExp) matching on the subject? I think that would be a good addition"
Yes it would be, but don't think it's going to happen any time soon, if at all.
I really think -- pardon my giving advice that you never asked for -- writing some scripts to run on a $5 / month VPS (such as Digital Ocean or whatever).
Personally I'd do it in Python, it has an easy to use IMAP library.
Or maybe consider If This Than That:
https://play.google.com/store/apps/details?id=com.ifttt.dobuttonOr another "automate my computer stuff" product.