Reasons to buy Canon or Nikon:
1. Widespread availability of parts, accessories, lenses, etc. Anywhere in the world you'll find what you need for Canon and Nikon. Not the same for Sigma, Olympus, Pentax, etc.
2. Community support- Canon and Nikon's user bases are huge. Any problem that can come up has been documented and probably solved.
3. Neither brand is going to go away. Canon and Nikon's commitment to the DSLR is tied directly to their company's success, unlike companies such as Kodak or Fuji who have since decided to stop production of their DSLRs.
4. Quality doesn't suffer despite mass production. A prime example of this are the Canon L-series lenses. It's very hard to find comparable glass whose use is so widespread while at the same time maintaining near-universal agreement among professionals and amateurs of their quality.
Yes, other companies have unique aspects or features that might be desirable, but when it comes down to what's important Canon and Nikon just do everything else a little better. Recommending one of them might be a result of personal bias, but at least it's not going to have negative consequences.