To put an end to all wild speculations, the USB requires that a device is the Master and the other one is the Slave. This is hardwired, so if a device can connect to a PC (which by definition is a Master) it cannot connect to a Slave, via the same USB port. This is also why one cannot directly connect 2 PCs via USB
So, I assume, the MicroSD writers/readers are for using with the PC, then the written/read is put into the eReader.
BTW, many eReaders with MicroSD slots act like cardreaders when connected to a PC, therefore one can directly write upon that card. In fact, all devices I have with SD/MicroSD/CF and the like, if connectible to a PC, act like cardreaders/writers. I have MP3 players, Videoplayers, photocameras, camcorders, even SAT-receivers.
Therefore, the above links are
false advertisements, selling unnecessary thing to uninformed/mislead people.