I too feel it's more of a grammar point than the usual form of expression we'd include, however I still think it's useful as an entry. That's why I approved it initially. I think it can stay.

I disagree, I do not see how 助詞＋の would be any more obvious to a beginner than any 助詞＋は combination - in fact, I would say it is less because where as では is often introduced earlｙ with ではない, での, からの and ての are not. 
If the dictionary hopes to be useful, it will need to cross over grammar and morphology at times, the grammars of english and Japanese are just too wildly different for many expressions to be taught in isolation - and yet, that is not the case here, all we have is a translation, we are not mentioning grammar nor morphology within the entry.

And again, we have many entries analogous to this situation in the [exp] tag：そう言えば・1982210、くじを選ぶ・2406890、その人・ 2832864、その事・2665570、ごく普通・2861349、ずっと前に・2038440、す可き・1006200、are all the sum of the parts, not in any kokugo I own, and are still included within the corpus. And those are just a few I gathered from the く up to そ pages.

では, をば and からに are in the kokugos and have non-obvious meanings whereas での is just the sum of its parts. I don't think many beginners would have difficulty parsing it in context.
To me this feels more like a grammar point than an expression that belongs in a dictionary.

I think it's useful for beginners, per Non. I don't it's reasonable to just show a beginner our entries for で and の and expect them to figure out what での means/how it's used. Maybe we should have entries for からの, までの, からへ and への as well.

”五人でのお茶会”　= Quantificational delimitative
”3時間での小旅行” = Temporal quantificational delimitative
”器具でのご使用” = Instrumental


Note: none of these sentence fragments are my own, they were pulled from various different google results of "での"

We do have entries for combinations of particles, では, をば, からに, etc. are just that. What is true is that we do not have 格助詞+の constructions, but we do have multiple entries which are just inflected terms made from identifiable morphemes that are analogous to this situation.
Personally, I think this entry might have some value for an unexperienced person who runs the risk of thinking it is a single morpheme and going off on a wild goose chase for some obscure particle called "での".
Consider, for example, that students are often taught that の attaches to nouns with no mention of the possibility of these particle compounds.

Now, I realised that this entry was incomplete as で's delimitative and instrumental uses can also be used attributively. I added [2] to cover the instrumental uses and [3] to cover the delimitatives but I am not sure how clear the first gloss of [3] is.
Maybe we could drop [3] by saying that something like「五人でのお茶会」is an extension of [2]: "Tea meeting by 5 people" = "Tea-party of 5 people" but it seems a bit of a stretch for something like "30分での判定".
Alternatively, all glosses could be replaced with an explanation of the etymology and cross references, that should be enough for the reader to assemble the meaning by themselves while avoiding any confusion.

@@ -10,0 +11,10 @@
+</sense>
+<sense>
+<pos>&exp;</pos>
+<gloss>(that is done) with</gloss>
+<gloss>(that is done) by</gloss>
+</sense>
+<sense>
+<pos>&exp;</pos>
+<gloss>(that is) of (a quantity, number)</gloss>
+<gloss>(that) spans (a certain time)</gloss>