The vast majority of applications do not pick their source address, nor is
there a compelling reason for them to do so.
not in IPv4. if IPv6 has site locals more applications will need to do so.
A large number of applications
don't even handle multiple destination addresses properly, so expecting this
additional intelligence for the source address is irrational.
again, you're trying to extrapolate from ipv4 experience to ipv6, but such
experience isn't necessarily a good indicator when ipv6 presents a more
challenging environment than ipv4. then again, if dealing with multiple
addresses for a host was hard in ipv4, it's not going to be easier in ipv6.