As a DM I feel summonings of creatures should be more usable at low levels but not as they are structured now.
The One round casting vs 1 round/level duration takes it out of most consideration before 5th level.
On the opposite side, the multiple creatures options and the choosing the creature you need in the specific situation are very good at higher levels.
So here is what I see as a different Spell option, not a replacement:
Chosen Celestial (Specific Creature) Summoning I-IX
School conjuration (summoning) [Good]; Level bard, cleric/oracle, sorcerer/wizard, summoner, witch; 1 to 9.
Casting Time 1 full-round action
Components V, S, M (a specific, low valued gift to the creature)
Range close (25 ft. + 5 ft./2 levels)
Effect one summoned creature
Duration 1 round/2 level (min.1) + casting attribute bonus (D)
Saving Throw none; Spell Resistance no
Like the Summon Monster spells, these spells summon one specific extraplanar creature specified from the same Summon Monster spell list with the Celestial Templates. The caster must have a Good alignement to cast the spell.
Ex.: Chosen Celestial Eagle Summoning I is a 1st level spell. Chosen Celestial Pony Summoning I is a different 1st level spell.
(Elemental, Entropic, Fiendish and Resolute versions would also exist)
Main differences with Summon Monsters:
Big Plus: Casting time Full-Round action vs 1 round.
Minuses: No adaptative versions of summoning, never more than one creature per casting and caster must meet alignments templates (elemental remain an option for every caster).
Plus and minus: Duration 1 round /2 levels + caster atribute vs 1 round/level.
Generally, summons are quite a strong option. The one round casting time is there so that they are easier to disrupt. Making it a full round is the same as a standard action in that sense.
Since there is really only one or two "best" summons at each level, I don't think being forced to choose when you prepare spells is really a drawback.
Further, 5 rounds is basically "all combat" so I feel your option is a straight up buff to one of the best spell lines.
Really, I feel summons are good from level 2 on-wards.