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...
