I never understood why those in camp (2) don't have a proper Google account not linked to Google Apps, and just forward all their mail along? That seems like the brain-dead obvious solution (and means that if you ever want to move your domain name or get rid of it a few years down the line, you always have a permanent address to fall back on).
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.
i'm sure an organization wouldn't appreciate their users forwarding company emails to a personal gmail account. in almost all cases, it is probably against their policy. it would actually be a burden for the user to have their email configured the way you describe, just to get access to a couple consumer features before they are hardened for enterprises.
