Where a reason is given in a verse, even Rabbi Yehudah would qualify the halacha based on circumstances where the reason applies. (This is true if the reason is explicitly related to a commandment). An example of this is the case of the number of a king's wives (Sanhedrin 21a).
Where a reason is not explicitly stated along with the commandment, Rabbi Shimon would nonetheless expound based on the underlying reasoning of the verse if a reason is implied by the verse. A primary example of this is the case of the widow's garment (Bava Metzia 115a). Since the verse prohibiting taking a widow's garment as collateral follows closely after the verse requiring one to nightly return the garment-collateral of a poor person, Rabbi Shimon infers a connection.
This connection is that returning the widow's garment daily must be forbidden for some reason, namely that going to the widow's home every morning and night will generate rumors and ruin the woman's reputation. Since this would only happen in the case of a poor widow, Rabbi Shimon expounds that the prohibition only applies in such a case.
The given example with Shabbos is not relevant anyway because "ki sheishes yamim... shavas vayinafash" does not mean resting from hard work but from constructive work. It therefore could not possibly be used to limit lo s'va'aru eish to situations where it is hard to make a fire/light.