You can uninstall LibreOffice if you wanted to (would free up over 130 MB), but it'll also want to remove ubuntu-mate-desktop - It wouldn't break the system, it's simply a meta-package that acts as a list of all packages that come pre-installed with the Ubuntu MATE Desktop. 
The only possible problem is during upgrades where new applications for that release wouldn't be added (I believe). Other than that, I had no problems leaving ubuntu-mate-desktop removed between 15.04 to 15.10. 
Since the entire suite isn't installed, trying to remove libreoffice isn't enough, you'd have to remove libreoffice-writer, libreoffice-impress, libreoffice-calc, etc. The Synaptic Package Manager is good for this.
As for installing OpenOffice (for reference), I can't see any packages in the repositories... so the user would probably have to grab a copy from their website. 