I'm pretty sure that all of those other sampling rates up to 192kHz are for playback only; not recording. If it can only record 44.1 and 96, and not even do that rate with the external memory, then I think this recorder is not a great choice for what we do here, compared to what other choices are out there.
It makes sense from an engineers point of view, especially in the light of the DR70D media hulabaloo, to use internal memory for high bit-rate recording. It's a known bandwidth/performance device. Take the variable away so it just works. With the price of memory dropping, I'm hopeful that this trend continues.
In pro video you can see the variable argument in the approaches of Blackmagic Design and AJA video recorders. Both use large removable media. The Hyperlink from BM uses commodity SSD drives, but with a recommended list. AJA sells their own drives, now 1TB SSD with USB3. Guess which product I've heard stories about involving hard to reproduce record errors?
Also, guess which company has the better warranty policy. Which is rarely needed. (Overnighted replacement after the first phone call with knowledgeable engineer, just send problematic unit back. Out of warranty? They attempt to fix it for free, and apologize for charging you shipping.)