Sadly, it's pure about money as if they wanted a better driver they have Klien signed as a reserve/test driver as well as Yamamoto. Yamamoto brings millions though.
[edit]
5liveF1 on twitter confirms that Chandhok will be joining them again on Friday for Free Practice 1 and 2
Shame I am working and will miss the opportunity to hear him during Free Practice again. As I mentioned in the German GP thread, Yamamoto has done nothing but make a fool of himself and the HRT management for trying to pass it off as driver testing/evaluation. I honestly can't see how any amount of sponsorship cash can gloss over how poor he has been. I am all for giving him ago but he has had is attempt to impress and failed.