One overlooked part of summons is the sometimes rather extreme skill levels they have. Just saying.
Which gets even better with Evolved Summoned Monster because +8 one skill is a 1 point evolution.
Maybe even more overlooked are spell abilitis, e.g. with summon good monster feat:
summon monster 2 =3 pyrotechnic/entangle(grig) or 1 hideous laughter (faun)
summon monster 3 = aid for entire party (latern archon)
summon monster 4 = aid for entire party, circle vs evil (hound archon) or permanent image (pixie) ("cheating" 2 spell levels)
summon monster 5 = blur entire party, 2 cure serious (azata, bralani) or 1 wind walk, 1 create food/water/wine, 1 gaseous form, 1 major creation, 1 persistant image (djinni) or 3 cure light, 1 cure moderate, 1 neutralize poison (unicorn) or 3 dispel evil (mostly vs enchantments), 3 remove disease (Agathion, Vulpinal)
summon monster 6 = align weapon, versatile weapon (Archon, Legion), hallucinatory terrain (Azzata Lillend)
Higher summon monster give only heal and freedom of movement as far as i can see.
While these spells are sometimes not that great, you get them for free by having the respective summon monster prepared (actually all spells of lesser summon monster level are avaiable through one of higher lebel). And some of these spells are useful and not on every list.