Vector images are most definitely preferred because of the scalability. The logo vector was already in the git repo, the text file is missing.
I looked on my 17.04 Ubuntu Mate installation(s) but I don’t have the ubuntu-mate-text.svg.
richard@Tully:/usr/share/ubuntu-mate-welcome/img/welcome$ ls -l | grep ubuntu-mate
-rw-r–r-- 1 root root 5806 jan 18 17:50 ubuntu-mate-gray.svg
-rw-r–r-- 1 root root 5890 jan 18 17:50 ubuntu-mate-icon.svg
-rw-r–r-- 1 root root 148096 apr 20 00:24 ubuntu-mate-laptop.png
richard@Tully:/usr/share/ubuntu-mate-welcome/img/welcome$ dpkg -l | grep welcome
ii ubuntu-mate-welcome 17.04.12~zesty1.0 all Welcome screen for Ubuntu MATE
Is it a old image from a earlier version of welcome?