In all honesty, this game mode does hold a ton of value for most players: especially those new players and those learning certain champs. Its just that no one really figured this out. DOMINION "WAS" A GREAT GAMEMODE FOR NEW PLAYERS, AND EXPERIMENTAL BUILDS!!!
Here are the things that dominion provided:
Because of a fast turnout, fast gold pace, and fast fights in general (constant, constant teamfights) dominion was a great place to feel out champ mechanics without being stuck in a 45minute (especially if you decided to try out a champ, only to do horribly). Seriously, compare this to dom where each dom game lasted anywhere from 20-30 minutes tops. Not only this, but it was a safe alternative for players who wanted a quick match but didn't want the casualness of aram or the long strung out games of SR. The fast pace of this game mode also allowed for quick game turnouts, and if you got stomped in a game you could simply que up again fairly quickly.
-Safe, PVP-practice mode
Dominion was a great place to feel out champ mechanics in a fast gamemode. The constant teamfights also meant that you could develop your skills in teamfights (which aren't emphasized in the regular SR). Often, i see people with alot of skill in lane, but poor teamfighting mechanics. The irony is that these same skills could have been developed in this game mode had riot marketed better. Also, if you wanted to practice ur solo lane mechanics in a quick environment you could go botlane and develop your skills there against ACTUAL players. You could also develop them quickly, as the quick game turn-over meant that you could test different starter builds (3-4 items) against real players and see if you could snowball with them.