When you create a structure, there is no problem to have as keys numbers. Example:
<cfset myStruct = StructNew()>
<cfset myStruct.key1 = 1>
<cfset myStruct.2 = "asd">
<cfdump var="#myStruct#">
If you will run that code you will see it working with no problems.
But let's say you have a situation where you need to have the keys dynamic. In this case i would have a script like this:
<cfset myStruct = StructNew()>
<cfset myVar = "key1">
<cfset "myStruct.#myVar#" = 1>
<cfset myVar = "2">
<cfset "myStruct.#myVar#" = "asd">
<cfdump var="#myStruct#">
And yeah... this one will give you an error like "The string myStruct.2 is not a valid ColdFusion variable name".
P.S. Before someone to point that you can have it as
myStruct[myVar], yeah, i know that 
But the situation where i needed it was when you have a deep structure with many levels and you don't know exactly what level you need to address. Like if you would have a function used to set a value for a given structure position (like SetStructure("myStruct.myStruct2.myStruct3", "some text as value") or SetStructure("myStruct.myStruct2.myStruct3.myStruct4", otherStructAsValue)). Having each level of the structure being a "valid ColdFusion variable name" all will work fine. But as soon as you will want a level to be for example a number, you will get into trouble.If anyone have a solution to this, i would love to know it too.
If the variable does NOT yet exist, you can use CFParam to hack it:
<cfset key = 2 />
<cfparam name="mystruct.#key#" default="asd" />
But, this ONLY works if the variable doesn't yet exist.
hmm nice hack Ben!
It of course also will work if to give any structure depth like
<cfset key = "rgeg.wete.2" />
<cfparam name="mystruct.#key#" default="asd" />
Did not think to use the CFParam hehe
Thx again
So I have to ask - why not use structGet?
<cfset s = structGet("a.b.c.2.d.3.f")>
The setStructure function you imagined above is almost the same.
@Ben and Ed,
why not just use structinsert with the overwrite set to true. Then you don't have to worry about deleting the key or throwing errors. You can ever use dynamic variables like so:
<cfset structinsert(mystruct, myVar, "asd", true)>