Posted By dan hilts
The sun is plenty bright enough that you don't require a large aperature to view. The off axis mask allows a smaller filter that is less likely to be damaged and is easier and cheaper to replace when it becomes necessary.
Also note that unless you have extremely good circumstances, the heat of the sun causes localized atmospheric conditions that destroy seeing. The effect of this is typically much worse than the loss of contrast due to the spider and seconday obstruction. There is little to be gained by using a full 10" aperature.