*Response time - ok, but thats pretty minimal
I couldn't disagree more. That's why I listed it first. I can arrow down through 30 emails glancing at the first line in 30 seconds. No chance in webmail. Unless of course you have a kick ass webmail I've not seen.
*Attachments - a peeve of mine*, but drag and drop can sometimes save time.
Your rant at the bottom is logical but as you know internet mail is a tinker's nightmare. It's a hodge-podge of standards some of which, including SMTP and mail-related DNS, couldn't have remotely contemplated what it has become. I tell you what. Get my boss to quit sending out attachments and I will too. For that matter, get the COO or IT Director to do so. Good luck.
*Offline - a pretty limited use case, and one I thought of before responding the first time - fits in my 5% rule.
At home or office, yes. Laptop on the road, no. On a plane trip I can create a dozen drafts and kick 'em all out the door when I land. Sames goes for a bus or commuter train. Not huge for me as I don't travel much but for some it's huge.
*Security - do you mean encryption? cus both can do that. Other than that I disagree - having any local storage is inherently less secure. I just received my third letter this year from yet another financial institution that had a laptop stolen and customer data compromised.
Local storage is not inherently less secure. Physical security is one page one of the security handbook and wraps up the epilogue to boot. Having my private personal or confidential business email available to anyone who can gain my password is a huge exposure. I'm not saying people don't leave their laptops in their backseat or expose their home network to anyone. That's why I included a caveat in my response. However, I'm very happy that I control where my data is and who has access to it.
I recognize that you're coming from a different perspective than a small company or individual. Your corporate webmail may be housed at NSA but most of the world uses a service. If that service's security is breached, or if a credentialed employee at that service decides he wants to read everything I've ever sent, I'm toast.
*Search - I have this too.
I submit not as fast, customizable or eloquent as mine but that's probably open to debate.
*Multiple accounts - ok, I can have multiple, but when responding to email I cannot change how it appears to others (to make it look like it came form the original account).
I mean multiple accounts in one place. I can't merge my Google mail with my Yahoo mail with my AOL mail. On my desktop I have all my accounts in one place. I can filter and drop according to my wishes. One login gets me everything.
Oh, one more thing, forged kicks ass.