I’d like to add my own 2¢, if I may.
I thought of starting a new thread, but I think my idea fits quite well into the current discussion.
How about making auto-selecting signatures more robust? I mean like: filter-grade robust. The thing is, I think everyone has on his contact list people with whom he has different kind of relation. Some of them are friends, so in their case some nick-containing, familiar signature is fine, but some contacts are “official”, so more formal signature is in order. It may be said that one shouldn’t mix his private and professional mails, but sometimes it’s just more practical to do so. At the same time remembering to select the right every time is a bit bothersome, not to mention error-prone. So…
How about giving signatures a filter-based conditional applicability? It may even be a part of filtering, rule-processing functionality, but instead of e.g. “Apply tag” there would be an action like “Apply signature” or… “Apply template”.
About the OP use case, I think it could also be done in filtering section, but this time it would require defining some new conditions. So beside “From” or “To”, there would be “When replying”, “When replying to all” or “When recipient in” (e.g. “When recipient” “is in” “”). Then there could be “If message has attachments”, “Contains a text matching regex”, I think that explains it quite clearly.
It seems quite comprehensible to me, not too difficult to implement and yet waaaay more powerful than any “Use this signature when writing a new message” and “Use this signature when replying” pair of options.
As an added bonus, the very same functionality may be utilized by templates as well. Just imagine having different templates applied for different recipients automatically. Bam! Done! :}