I don't know about you, but I'm using the C.Falco PPA in Ubuntu and the way they set up the defaults is a little bit confusing because you don't always have write permissions to the directories outside of your /home/.
.deb packages are intended to be sytem-wide, i.e. they must take into account that multiple users could take advantage of them, even games or emulators. So
- /usr/share/games/mame/roms is there in case some free rom will be bundled with Mame in the future
- /usr/local/share/games/mame/roms is the best place to put your roms if you have two daughters playing Mame with their own user and you don't like to waste hd space making different copies of your full rom set

- $HOME/.mame/roms is what hard-retrogamers are looking for

Please have a look at
/usr/share/doc/mame/README.Debian, some other good infos there.
To avoid all these problems my /etc/mame/mame.ini file only has one line in it:
That way it always points to the .mame directory in my home.
There's no need to tamper with /etc/mame/mame.ini for this. That's exactly what it does by default.

inipath $HOME/.mame;/etc/mame
If you remove the trailing /etc/mame your only achievement is to loose the path to presets ini files for some systems bundled with Mame. You may not need them, but other users may do.

One final trick since I've started using the default UI more these days; rename the /etc/mame/ui.ini file to something else, and then create a new ui.ini file under your /home/xxx/.mame/ directory and use that file to define all of your UI search path options.
Again, you don't need to change anything in the /etc/mame tree. Mame scans your $HOME/.mame/ directory
first and goes on if nothing is found there
only.
The multiple-semicolon-separated-path is a basic MAME configuration feature, and it's not confusing at all once you grasp it.
