Some people use many Bitcoin wallet specially those people who wants to earn a lot of bitcoin every week for signature campaign plus many accounts here in bitcointalk. As for me i only use 1 Bitcoin wallet , 1 is enough for me to received and send bitcoins.
Probably some are doing that, but not all... There are people who are always after the security of their investment... It's a good practice, who knows, your computer might be compromised or the site that you are logging in is compromised so there is a chance that the wallet you've used there will also be compromised...
Yes. Just like changjng passwords of email every month. This should be done regularly. Look what happened to yahoo. The news spread that they have already been hacked few years ago and they just knew it now as they want to sell it. The thing is many people now are getting good with computers and can make applications that can hack accounts faster than you expect it. Just one visit in a website can change it all.
That was certainly true. It's something very terrible if could not anticipate. Therefore we can do harm if they take is personal or confidential data which has a very good value.