What does an external video recorder like a Ninja offer?
If you record (typical AVCHD) sub-30 Mbps 4:2:0 video to a card and 220Mbps 4:2:2 video to an external recording device, you will be hard pressed to tell the difference when you view it back on YouTube.  Very, very hard pressed to tell the difference.

However, if you plan to edit the video--changing contrast curves, color, saturation, etc., and especially if you are trying to key any region (whether for green screen replacement or because you want to muck with certain color ranges in certain ways), or if you want to digitally zoom in a bit, then you are going to immediately notice a large difference in how the video behaves in your editing program.

What makes the low bit-rate recording work so well is that the compression algorithms are very smart about fooling our vision system, which is very easy to do.  But such smarts break down almost immediately as soon as you start treating the video as a series of images to be manipulated (which is how video editing programs ultimately "see" the video).  That's when you are going to wish you had recorded in a format that is more true to the digital workflow and less clever about how easy it is to fool our vision system.

If all you ever want to do are the simplest of cuts and fades, you can do that with low bit-rate video.  But if you want to push the image around more robustly, you will quickly realize the need for a higher bitrate solution, which is what the external recorders give you.