Plus the differential versions of most of those signals for long distance doubles the number of pins. And they have optional synchronous clocks. I did some WAN work for 3Com back in the day... :)
A few devices back in the day could be powered over DB-25 connections besides the communications, even. Some in factories might still be.
If I recall correctly, the DB-25 RS-232 cables also facilitated two serial connections.
You do. (I've never seen that used, though.)
Yes and then we came up with soft flow control for the win