The Security Side of Me:
One of the companies (MediaDefender, I think, anyone want to confirm?) that was helping the RIAA track down and prosecute Bittorrent sharers permitted one of their VPs to forward his company mail to his Gmail account.
Some people who did not appreciate this company's efforts guessed his password and pulled all his mail, and leaked all the company-related mail. You can probably still find torrents of it floating around. Included were salary documents, operating plans, etc.
So, you need to not only trust Google, but also trust the developers to pick reasonable passwords. You might think that, at least once it's explained, it would be a simple matter to get the developers, who should all be "computer people" (but sometimes aren't) to pick good passwords. Anecdotally, however, I've seen many developers with piss-poor security.
The User Side of Me:
On the other hand, I hate having multiple mailboxes, and I get much better productivity out of having all my mail flow into one. I check it all in one place (and all the bloody time); and I don't really have to care where people email me at, it all reaches me eventually.
As we speak, actually, I'm doing a mail migration of several Gmail accounts into one GAFYD account, and after that I'll be pulling in my old Outlook/Exchange archives.
If I couldn't forward my work mail into my GAFYD account, I'd be very peeved, and you (the person stopping me) would be rather screwed anyways, because I would be importing my archives instead. Which brings us to the bottom line:
The primary reason not to let me forward my mail into Gmail is security. And if I want my mail in Gmail bad enough, I'll get it there one way or the other. Maybe it won't be in real-time, but I will eventually, and once it's there, it's the same as if you let me forward it in.