Question: In [Kiddushin 34a:4](https://www.sefaria.org/Kiddushin.34a.4?ven=Daf_Shevui&lang=bi&with=all&lang2=en) in the context of hakhel it states

> 
> אָמַר רַבִּי יוֹחָנָן אֵין לְמֵדִין מִן הַכְּלָלוֹת וַאֲפִילּוּ
> בִּמְקוֹם שֶׁנֶּאֱמַר בּוֹ חוּץ R. Yohanan said: We cannot learn from
> general principles, even where exceptions are stated.
> 
> 
> 

For limud torah, what's the epistemic value of general principles, i.e. what would be their use / purpose if one cannot learn from them (even scoped w exceptions as above)?

Answer: 
The gemara goes on to give an example of a general rule that has an exception, demonstrating that they aren't intended to be reliable ways to derive halacha. Therefore you cannot use a general rule as a proof to resolve a safek.

What, therefore, are the uses of a general rule (a Klal)? There are many. It is often used as a starting point in a discussion. It is often given purely as a memory aid. The rule itself also teaches you something about how the halacha works, the will of Hashem, etc.

To directly answer your question about epistemic value: it's also necessary in conjunction with a specific rule (Prat), in Rebbi Ishmael's [13 talmudic rules](https://judaism.stackexchange.com/questions/10873/where-can-i-find-examples-of-r-yishmaels-13-talmudic-rules), 4-7:

4. Klal U'Prat
5. Prat U'Klal
6. Klal Uprat Uklal
7. Klal Shehu Tzrich Liprat

as well as:

8. Kol davar shehaya bichlal...
It doesn't mean that don't learn from them at all or that they aren't true as a rule. It just means that a general rule is not to be taken as such an absolute rule as to assume there no exceptions to it. Not even if the general rule itself made exceptions because it could be there are more.

That would be true about general rules that are given about almost anything. The rules serve as a starting point and an organizing principle from which further understanding, while we delve deeper to understand the exact and specific applications can begin.