Can't we all use multiple, personally, I use WP, PHPBB, and Tikiwiki. . . simultaneously, I'm still working on making them all come together, bu I find that many specialised programming sets are best, just like having one type of employee at an office just doesn't work, a full-feature CMS is easy enough to replace with WP and PHPBB and a wiki software, I chose WP for its huge commmunity (though Nucleus and movable type both looked promising), I chose phpbb because I found it a head-and-shoulders above SMF, and I hadn't had a site for more than a day or two, so I didn't want to go out of the Cpanel range, and SMF was ungodly, I culdn't manage to change themes, the IP addresses of each member were practically broadcasted around without an easy stop, and the whole site looked like the homepage of a spam site : P, in retrospect, phpbb is definitely the best free thing around. As For my choice for Tikiwiki, I found that it was:
easy to customise (duh, it's a CMS, Right?), well not in that way, what I meant was easy to Theme, though rather risky, I crashed it twice trying to get add-to-any on there (not to mention 2 or 3 WP crashes from wp-united and another plugin I had grinding teeth,(plus the update to 3.1 threw the whole system off) (no phpbb crashes, it had very good safeguards for when things broke, it sort of limited he carnage to one, ugly spot), anyways, back to Wikis, phpwiki looked a bit limited, Zikula was very confusing to me at the time, and Drupal looked promising, but was banned by parental controls. . .

Moral pf the story, using one CMS for an entire site is a little dangerous, if I had, my ENTIRE site would have crashed. . .
Multiple Times
(of course, my site was my very first site, and I have only been a player in the internet since December 29th, 2010, when I installed phpbb, and learned CSS, PHP, and HTML through it, without a crash, and then moved to WP, which was fine fora while, then I moved on to Tikiwiki, a tad unstable, and then I tried merging all three, screwing up WP and phpbb in the process, but phpbb's issue were very easily fixed, WP's were a little more troublesome and yeah, that was a summary of my entire site. . . so far)