Summon Monster for Equivalent CR?

I think I read somewhere from a WotC source, that a variant rule for Summon Monster would be that a Summon Monster spell of a particular level, could summon a monster of equivalent CR.

For example, a summon monster 2 spell could summon a CR 2 creature. However, the source said that it should be handled on a case by case basis, since some monsters may be unsuitable for summoning purposes.

Being the DM of my game, I was thinking of having summon monster spells summon NPC warriors. For example, a summon monster 2 spell could summon a 2nd level Warrior Half-Orc from some other plane. I remember in the old Planescape product, one of the hassles for adventurers were "summoning crystals" that would come at inopportune times and summon adventurers away for a couple of rounds or so.

Anyways, would this be unbalancing?

I remember a Dragon magazine article that suggested the following CR-spell level conversions:

Summon Monster 1: CR 1/2 or less
Summon Monster 2: CR 1
Summon Monster 3: CR 2
Summon Monster 4: CR 3
Summon Monster 5: CR 4
Summon Monster 6: CR 5 or 6
Summon Monster 7: CR 7 or 8
Summon Monster 8: CR 9 or 10
Summon Monster 9: CR 11 or 12

I'd probably disallow humanoids and giants as summoned creatures, unless you're running a silly campaign.

Squire James said:

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I'd probably disallow humanoids and giants as summoned creatures, unless you're running a silly campaign.

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I don't see why humanoids and giants are silly...

Personally I think they're some of the safest monsters to use.

An important point: Stay away from allowing monsters that are lame, but have some high-power ability that doesn't effect combat much. An example would be that thing in fiend folio (I think) that can open planar gates, but has an extremely low CR.

Squire James said:

I remember a Dragon magazine article that suggested the following CR-spell level conversions:

Summon Monster 1: CR 1/2 or less
Summon Monster 2: CR 1
Summon Monster 3: CR 2
Summon Monster 4: CR 3
Summon Monster 5: CR 4
Summon Monster 6: CR 5 or 6
Summon Monster 7: CR 7 or 8
Summon Monster 8: CR 9 or 10
Summon Monster 9: CR 11 or 12

I'd probably disallow humanoids and giants as summoned creatures, unless you're running a silly campaign.

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I've used something like this, though it does tend to break down at the higher levels. It's not very useful if anything you can summon at 17th level is just CR 12, and thus dead in one round by anything you're going to be fighting. I realize that there are higher-level summoning spells, but I think Summon Monster should remain useful. It's a tough call, though, because higher CR monsters have all kinds of abilities, which can make things tricky. If it's just being used to summon a warrior-type critter, I usually use something along the lines of...

(Min CL 1) Summon Monster 1: CR 1/2 or less
(Min CL 3) Summon Monster 2: CR 1
(CL 5) Summon Monster 3: CR 2
(CL 7) Summon Monster 4: CR 3 to 5
(CL 9) Summon Monster 5: CR 6 or 7
(CL 11) Summon Monster 6: CR 8 or 9
(CL 13) Summon Monster 7: CR 10 or 11
(CL 15) Summon Monster 8: CR 12 or 13
(CL 17) Summon Monster 9: CR 14 or 15

I always do custom summoning lists for characters that actually use the Summon Monster spells, since I find it much more interesting. So far, I've had a dwarf cleric who summoned celestial dwarven warriors, an elementalist who summoned magma-related creatures, and a cleric of the goddess of illusion who summoned illusion-based things. And right now I'm playing a mystic theurge focused on undead, who has a summon list full of undead critters.