Videomaker – Learn video production and editing, camera reviews › Forums › Technique › Sound › Why use a external recorder? › The principal reason not to
The principal reason not to record directly to your camera is the generally poor quality of in-camera preamplifiers or "preamps".
Whether your microphone is on the camera or off, the noise contribution of the crummy preamps in most DSLRs (even those with manual gain control) is apparent in quiet scenes.
The H4n has marginally better preamps than a DSLR (still noisy by professional standards), the JuicedLink preamps are quieter than the H4n and the preamps in the professional quality $2000 Sound Devices 702 field recorder are quietest of all.
Here's a great explanation/comparison from Robert at JuicedLink:
Hope this is helpful!