We've had a number of discussions about cropping. The thing to remember is cropping is not free - there are consequences. I you are cropping you need to make sure the image is very sharp and keep your ISO down. These can be difficult parameters.
If you crop from 51 MP to 3 MP you are cropping to 6% of the original image. But any noise in the image is still the original size - and magnified in relative terms to look 15 times larger. At the 46-51 MP range, I can shoot with a top ISO of 12,800, process normally with DeNoise or similar products, and end up with a high quality image. But when you crop to 3 Megapixels, you are magnifying the effective noise by about 3.5 stops. So rather than shooting at ISO 12,800, you need to shoot at ISO 1200. (Look at Photonstophotos.net and compare an FX camera with a DX crop from that same camera - It's about a full stop difference). Trying to do that with an f/6.3 lens in anything but bright daylight becomes a problem. In addition, all of your edges are wider in relative terms, so the image looks soft compared to an uncropped image and it lacks detail.
Depending on the computer, a 100% view is typically around 1080 pixels. So a 100% view of an image would be 1080 x 720 or about 0.7 MP.
If I'm posting an image on the internet, I'm typically looking at 800 x 530 pixels or about 0.4 megapixels. So with a 3 megapixel image, you have some benefit of downsizing which reduces noise by about 1.5 stops, so that ISO could be around 3200 for we use, but an 11 x 14 print is going to require upsizing and an even lower ISO.
So there are plenty of good reasons to crop - especially for web use. But if you are looking at any kind of print - magazines, galleries, etc. - the same image would no longer work.
The practical side of this is that if you know the difference, you can make it work in your favor. A 100% crop handled well probably looks okay on the internet - especially if you keep the size small and benefit form downsizing.
The other thing you can do is shoot wider and make environmental images rather than head shots. Include the environment, multiple subjects, etc. so you can crop less. That may be a more interesting image anyway.