A spell-like ability takes one standard action to complete, while a summon monster usually has a casting time of one round.
So, when is the summoned monster of a Summoner's spell-like ability able to act?
same round as the spell like ability.
From PRD -> Magic -> Special Abilities:
"A spell-like ability has a casting time of 1 standard action unless noted otherwise in the ability or spell description. In all other ways, a spell-like ability functions just like a spell."
Plus, the Summon Monster special ability of the Summoner specifically says that it is a standard action to use it.
"Starting at 1st level, a summoner can cast summon monster I as a spell-like ability a number of times per day equal to 3 + his Charisma modifier. He can cast this spell as a standard action and the creatures remain for 1 minute per level (instead of 1 round per level)."
So, basically a Summoner can do the following actions:
Round 1: Summon a creature
Round 1: Creature makes a Full Attack action
Round 2: Summoner delays after his creature
Round 2: Creature (if still present) makes another full round action
Round 2: Summoner uses Summon ability to summon ANOTHER creature; Creature 1 disappears, since it cannot coexists with another one brought in existance with the same ability
Round 2: Creature 2 makes a Full Attack action
This can be useful if (for example) Creature 1 has an active Smite XXX ability against a target and kills it; since it has no more Smite XXX abilities, its power drastically decreases. So Summoner 'trades off' his own full-attack action for the Full Attack action of ANOTHER creature (which still has its Smite XXX ability). Same is valid for other abilities which have a limited use (like a Bralani Azata's Lightning Bolt SLA or a Trumpet Archon's Clerical spells).
And no, this is not a loophole, since the Summoner is consuming one of his limited Summon abilities per day to do this, and he is left with only a move action after Summoning the creature.
Okay, thanks for the clarification.