Hello there!
Although taking my first steps into the digital world having bought recently my first dslr (pentax k-r) i will share something i've learned while reading the forums here (and i hope i'm not making any mistake here).
related to the K-R but i think we can say this regarding any DSLR. It's true that 200 is base iso but i assume it's because at that particular sensitivity (and higher, ofc) the sensor can achieve the best dinamic range.
I remember seeing a few photos here on the site (will link the thread when i get home), where in terms of noise (at pixel peeping levels) the photo shot at iso100 was slightly better than the one shot with iso 200. So, in certain conditions, with plenty of light, you might want iso 100.
Something else i, for one, don't quite understand.. How is it that with a lower sensitivity and no camera post-processing aplied (HDR, Highlight correct etc) it seems you get more detail with a lower iso than with the higher value at this level (100 - 200 iso)
Quote from the k-r review here on this forum: "
ISO 100, however, does seem to pull out just slightly more detail in the shadowed areas comparet to ISO 200."