The internal oscillator is normally not as accurate than an external one. I have never had any problems with it, but of couse this won't help if in your specific case it's not accurate enough.
The other reason is that the internal RC is limited to 8 MHz. If look at the board you have mentioned in your question, there is a version for 16 MHz. So possibly from a design/manufacturing point of view it's easier to have one PCB, and possibly the same fuse settings for the boot loader. So it might simplify the manufacturing, and it possibly prevents complaints from customers who have problems with the internal one.