The DR-05(x) has a combination mic/line input with variable gain covering the entire range without switching. However, with the gain reduced to absolute minimum (gain cannot be reduced to zero), the input is still more sensitive (less headroom) than a well designed consumer -10dbV line input. The record level will go over 0dbfs (as in will digitally clip) with 500mV (0.5V) rms or more. This is dangerously close to the -10dbV standard level of 316mV (0.316mV). Most line level devices have much more output headroom than this (classic example being CD players). This makes things unpredictable for high output condenser mic sources. Depending on the mic sensitivity and battery supply voltage, the 05 input could easily clip. One advantage of using an external battery box is an in-line pad/attenuator can be inserted between the box and the recorder input to get the mic dynamic range to match more closely with the recorder’s input dynamic range/headroom.
By the way, the Roland R-07 uses a similar analog processing chip as the DR-05/07, and has the same issue with minimum gain. I built an in-line variable attenuator for a friend with an R-07 so he could tame the RCA line output feed from his DJ mixer to prevent clipping the recorder.