I've thought about this a bit more, and there is one situation in which your computer username and password are the same as your account. That is if your computer is part of a network; business, enterprise, education, etc. In that case you don't so much have a computer as you have a user on the network.
If your main email account is also being served by that network then it will have the same password. But that would mean that your email address is being provided by that network/business, not by an external service like hotmail, outlook.com, etc.
I suppose there might be a kind of halfway house sort of thing going on with a network that used Office 365 subscription services.
Anyway, it's possible I'm wrong about the computer vs online password. I don't think I am, because it sounds like you're using a personal computer, but if I am, I want to make one thing perfectly clear.
Having the account and password on the iPad only makes you less secure if you are walking around without password protecting the iPad. In that case if someone who gets ahold of your iPad they could look at your email. They could probably even use your email provider's password recover process to change the email password.
What they could not do is retrieve the password from the iPad. It's not stored in the clear. Once entered, account passwords on the iPad are encrypted. They can not be viewed or retrieved. Which makes it a real pain if you forget them and don't have them recorded (in a safe place) elsewhere.