Shard O'Glase said:
pathetically weak. In 3.0 they were a tad weak and there versatillity made them a good spell, in 3.5 there pathetically weak and there versatility makes them a pathetically weak spell. If the casting time was 1 standard action instead of 1 round then they'd just be slightly weak, as is though there a waste of space.
FYI: This opinion, though common, has not held by all players. Many of us have found these spells to be very valuable.
I think one problem is that people only focus in on one aspect of what they are gaining from a summoned monster. They ignore the rest of the details.
For instance, a SM V can be used to call a bearded devil (Barbazu). It will be around for at least 9 rounds. What happens when it is summoned for combat usage?
First: It can deal damage. A few strikes from a barbazu can result in massive amounts of damage due to infernal wounds.
Second: It can absorb damage. Every attack aimed at the barbazu is an attack that doesn't deal damage to a PC. They may only have 45 hit points, but they are pretty resilient to magical damage attacks (SR, immunity to fire, resistance to acid and cold, decent saves), damage resistance and the ability to rage itself to gain 12 more hit points. That is a lot of damage that can be absorbed.
Third: It can provide tactical advantages not available to standard PCs. For instance, it can do something suicidal. If the BBEG has a powerful one shot weapon, the barbazu can walk right up to the BBEG and take the hit.
Other monsters can also provide access to spell-like abilities that the PC can not usually create. A high level sorcerer that has a high level summon spell has access to hundreds of spells that may not have been on his spell list.
In the end, the spell is very versatile in terms of different abilities that can be gained and in terms of different uses for the spell that can be achieved through one casting. It is offensive and defensive at the same time.
In my experience, the only time the spell is useless is at low levels when the duration is so short that the spell duration ends before the creature dies from combat damage. Even then, it is often useful outside of combat.