I have deactivated Spotlight, because the mdb process used a lot of memory and I use Spotlight not really often :-) Today I searched some emails in Outlook (Office for Mac 2011) and the mails could not be found, but they are there (I can see them!). Is this only by accident or does Outlook use the Spotlight index?
As you suspected, Outlook for Mac does use Spotlight's index files (as do many other apps, such as Alfred). If you've got Spotlight disabled, search in Outlook isn't going to work either.
Spotlight shouldn't take up any memory really at all if it's not indexing and/or updating indexes.