Why is mount.cifs not installed suid by default?

I don't know the reasons for Gentoo but in Debian mount.cifs has the suid bit set and it is supported by upstream nowadays. To get the rationale and understand the reasons you should create a bug report at the Gentoo bug tracker - I would contain a link to the commit which enabled suid support again.

As it looks for Gentoo's wiki, they seem to be worried about its security:

http://en.gentoo-wiki.com/wiki/Samba#Non-Privileged_Mounting

They show you how to do it manually but also warn you about security risks.

Above that section, at first lines of page they also note the following:

Note: net-fs/mount-cifs, the old mount helper, is no longer needed, as the current stable version of net-fs/samba includes all of its functionality.

So you seem to have both choices but they recommend using samba, it has an USE flag 'client' so you don't have to install everything. (It's been quite long time without using Gentoo)