How to make a decision on the power level of conjured elementals?

Question

Conjure Woodland Beings allows the player to choose one option, ranging from a single fey creature with CR 2, up to eight fey creatures with CR ¼. But which fey creatures of the chosen CR actually appear, is up to the GM. This way the GM can avoid overly chaotic or broken outcomes (like 8 pixies). But the overall “effectiveness” of the spell should stay (roughly) the same, regardless of the creatures the GM chooses, because creatures with the same CR should be of comparable strength. In this case the GM decides details about the spells, but not the overall “power”.

Conjure Elemental on the other hand lets the player only choose the type of elemental (e.g. fire) and then “an elemental of challenge rating 5 or lower” appears. I assume that it is up to the GM to determine which elemental of the chosen type appears, similar to Conjure Woodland Beings.

In the given example, the GM could decide that a Fire Elemental appears, as it is of the chosen type and has a challenge rating of five. But the GM could also rule that an Azer (CR 2) or a Magmin (CR ½) appear. Both are “fire elemental” and both have a CR of 5 or lower. As a result, the “effectiveness” of Conjure Elemental is completely up to the GM.

Now my question: is there is any reason the GM should not always choose an elemental with the highest possible CR?

• If yes, how should the DM adjudicate the power level of the conjured elemental?
• If no, why would the spell specifically state “CR 5 or lower” and not just “CR 5”?

Answer 1

Give them the strongest possibility

(Unless they ask for something else specifically)

Conjure Elemental is a very situational spell as is:

• The casting time makes it impossible to use in combat on the spot
• When you cast it ahead of time, you risk running out of the 1 hour limit
• You can try to cast it just on the doorstep of the Big Bad, but you might get interrupted

As @Erik noted, it might become broken in the future, but currently giving anything but the CR5 versions can rightly be perceived as malignant.

Answer 2

The CR of "5 or lower" isn't fixed. When you cast the spell from a higher level slot, it will summon higher CR Elementals. While all the basic ones are CR 5 and fine, it might be the case (as with Conjure Woodland Being) that there is a CR 7 Elemental that is completely broken (or doesn't fit in the 10ft cube that is the area targeted) that you don't want to summon.

Alternatively, there might simply not be an appropriate Elemental of the given CR.

The wording as is, gives the DM the option to bring down something smaller or less broken, if required. I'll agree that when it comes to casting it as a regular 5th level spell though, using the regular Elementals seems totally fine.