Megapixel counts matter if you need to crop out large sections of an image & still need to retain enough pixels for a large enough print.
To illustrate Aydin’s point, I’ve taken a full photo of an Australian Monomorium nest and cropped it away to show just the queen ant:
Viola! Instant magnification. I can get away with a tight crop because the original photo spans over 6 million pixels. Blog photos only need 100,000 pixels. Plenty of pixels to spare. Once I get my hands on the Canon 50D, I’ll have 15 million pixels and the resolution to crop down to a single small ant.
So I can crop, but should I? Is cropping a good thing?
Bad habits are easy to get into, but difficult to break. If you get into the habit of cropping all your images then you’ll never put a lot of effort into composition or framing. Why should you when you can just crop it later, right?
While I am inclined to agree with Dalantech, I don’t think cropping need be detrimental to learning. I first learned composition through cropping. Once I got the hang of improving a photo by cutting out distractions and shifting the frame in post-processing, I started trying to get it right in the field using just the viewfinder. Cropping was a bit like training wheels.
Once I went commercial things changed fast. It turns out that photo buyers sometimes need a lot of pixels, especially for posters and museum displays. I can vouch for how embarrassing it is to lose sales because the image was too heavily cropped to be enlarged. Nowadays I aim to nail the composition in the viewfinder first, and if I do crop I do so sparingly, retaining at least 70% of the original pixels.
The persistent temptation in macro-photography is to crop for a bit of extra “magnification”. If you find yourself cropping as a matter of routine, it’s worth thinking about updating your optics. Think about it this way: If you are chopping half the pixels off your 10MP images, you may as well have bought a cheaper 5MP camera and spent the difference on a more powerful lens or a set of extension tubes. You’d get the same effective magnification but your images will be sharper and smoother.
Here is a shot of the same queen ant as in my example above, but with magnification augmented the old-fashioned way. I used a more powerful setting on the MP-E 65mm lens.
A crisper, cleaner image.