I forward my google account to my google apps. My main issue is that all my contacts are neatly organized in google apps and it's hard to use services like Google+ or Google Voice when they pull contacts from my gmail account.
For me the reason is integration: Calendar invites, Google Docs "shares", and other permissions grants don't forward, so they always end up tied to the Google Apps account. Which means using it as a primary account, gives one a better more integrated experience.
I used to forward from my Google Apps account to regular GMail but different people would see my email address from my domain and others from gmail.com depending on their mail client.
It's a minor nuisance but switching it around and routing everything to my apps account solved that inconsistency.
It's been a while but I believe when using the "Send mail as" feature in gmail it puts your preferred address in the from: field but keeps your gmail address in the sender: field.
That's probably fine in most cases but I was getting inconsistent results when people replied to my emails. Some were addressed to my domain and others to my gmail address.
I was also concerned that this might give me a slightly higher perceived risk in some SPAM filters.
Like I said earlier, none of this was a huge deal. It was just one of those inconsistencies that led me to routing all my mail to Google Apps.
