You certainly wont damge your saw buy doing so, but you also pay more money for a product that is of equal or lesser quality. In short better or equal products can be had for less money.
Why fool around and take the risk of using something cheap or different
Because in some cases the cheap oils are identical to the OEM oils. Take for instance Echo Powerblend. PB and Citgo aircooled share the same basic formula with the only differance being Citgo Aircooled doesnt have a fuel stabiliser in it. It also cost around $2 per quart. Another example would be Mobil MX2T. I can get it for between $3-4 per pint. Cheaper than Stihl or Husky oils, yet its is one, if not the highest quality aircooled, two cycle oil on the market.