1. I can't splice in other feeds from other services I use to into the default feed. Example, my feedburner feed includes links I post to del.icio.us. Posting stuff there is like informal blogging for me - it's quick, easy and I will normally pick out a quote that shows why I think link is worth a read (and/or provide short commentary). So if you're sub'd to my default feed you're missing about one half of my 'posts'. (note - more splicing options are hinted at by feedburner in their support forums - right now it's limited to a certain number of services, del.icio.us is one)
2. I can't redirect users from my default feed to my feedburner feed automagically (I would have done this already if I could). This blogware is good, but if I decide to change or experiment, you're stuck with my old feed.
3. The last reason is selfish (but ultimately better for you). I'm a stats freak (I've now have around 900 feedburner subscribers since September and still have around the same again for the default feed). The feedburner stats give me great feedback on what you find of interest (at an aggregate level). Now, since you've been kind enough to subscribe to me, the least I could do is for you is for me to give to you less of what you find utterly dull and more of what you like.
(There is a slight downside for me on this by having the fb feed. Stats get published internally on our blog / rss traffic, so because I'm directing users to my fb feed, these fb rss views don't count on the internal stats. But the above pros counter this niggly con.)
So, please, if you're not sub'd to this feed please update. Thanks.