I wonder for what that operation might be useful, but [Reproducible] with parallel installation of Master "LOdev 3.6.0alpha0+  – WIN7 Home Premium (64bit) ENGLISH UI [Build ID: 7175cee]" (tinderbox: Win-x86@6-fast, pull time 2012-05-16 22:07:37)

In attached sample you see 2 strings looking like numbers in B5 and B7 what are missing in chart, although for any calculation they are accepted as numbers. I can't see any sense in this behavior.

Created attachment 61783 [details]
Sample Document

I also see the described behavior with LibO 3.3.3 and OOo 3.2, the behavior seems inherited from OOo

@Kohei:
Please set Status to ASSIGNED and add yourself to "Assigned To" if you accept this Bug

Created attachment 61875 [details]
Illustration Graph with and without formulae

Attached spreadsheet illustrating problem - note that although number format selected, cells displays as text default

Reproducible with LO 4.4.0.0.beta1 and 4.3.4.1 (Win 8.1).

But the MID function is a text function (https://help.libreoffice.org/Calc/Text_Functions).
It extracts a text string out of a text and therefore the result is a text (LO Help: "Returns a text string of a text.": https://help.libreoffice.org/Calc/Text_Functions#MID).

Therefore, I would mark it not as a bug.

I thought about where there could maybe be a need to extract a number out of a larger number or text and I could there maybe think of a Personal ID Code or other generated codes where you need to extract a number.
In this respect it is interesting to see, that LO can already calculate with the text string result and if you add a number calculates a correct sum/number.
Therefore, I would mark it is an enhancement request that either the MID function can also return a number as one further feature or implement a further similiar function that returns a number.