I like randomizing the number of minor races.  They never break a game, but they add lots of opportunity for individual actions that shape the game.  If nothing else, it changes things according to who gets the resources they often claim but can't hold against determined aggressors.

They do get conquered a lot.  Sometimes it just doesn't work out that way and that adds to the variation from game to game.

Then again, the major races often get conquered in my games, so the minor races shouldn't feel so bad about being targets.

Minor races also buy the player "time" as the most aggressive AIs will generally go after them first, instead of the player.

On the higher difficulties especially, minor AIs are revenue sources in the early and early-middle game.  With tech trading allowed, minor AIs are also a source of many early game techs.  For example, they will always research a weapon, a defense, and life support.  They always want Trade as a tech, and will use it to send you freighters which will also extend their own "lives" as trade will slightly delay AIs from declaring war on them and conquering them.

Additionally, sometimes there is the event in which a minor race "steals" all the techs of a major AI race.  In DA, there will be the message that the minor AI wants to trade the techs for planets.  This is still bugged, as the AI will do no such thing.  However, if one declares war on that minor AI, sweeps away its vessels, and is poised to invade, that minor AI will trade many/all of those same techs in exchange for survival.  That tech windfall can be the difference in Suicidal games.

All AIs start off "knowing" where all inhabitable planets and mining anomalies are.  Major race AIs rush buy colony ships first, then constructors.  Since minor AIs cannot colonize, they start off rush buying constructors.  Hence, a player has a better chance of being first to mining anomalies when minor AIs are NOT in the game.  OTOH, minor race mining bases are ripe for vulturing.  Minor races allow some other somewhat gamey tactics, also.  When one has nine suicidal-level AIs in the game, one may become a bit more open-minded of such ploys!