We've only been at this for about 4 yrs but we have a small heard that we calve both spring and fall. My reasons were the same as you've heard, better options for sale barn prices, spread out the income, etc. One that you have not heard yet is that I am starting to sell sides/qtrs etc and hope to get to selling by the cut. Having two calving seasons gives me a better supply of products.
The issue of slipping one from one season to another is a plus and minus. My first bull performed well the first yr, the second I got 2 calves. (yes, I now preg check and BSE the bull - expensive lesson) I had a choice to cull all my animals basically or swith to fall calving. Plus, since I keep a bull, it keeps him busy more than 2 mthns a yr.
Roy