Getting past a locked home screen is a minor annoyance, not a problem.
Saving login/password combinations for games or forums is a minor problem if someone steals your phone. You can always change the password on the site and/or ask the admin to change your login.
Saving a login/password to, say, your bank account is a bad idea. It takes the thief 5 minutes to transfer a large amount from your account to his account, then to some offshore account, then to some account in a country we have no banking agreement with, then back to a usable account of his in the US. Now he has your money and there's no paper trail for the police to follow.
The general rule is to not save passwords for things you can't afford to lose. If someone got my login to Android Central, I could probably get it back in a day or so. I can afford to lose my forum account here. (I wouldn't like it, but I'd survive.) If they got my login to one of my larger 401k accounts (none of them are accessible online), I'd have to change my retirement plans. I couldn't afford to lose hundreds of thousands of dollars without a MAJOR lifestyle change.
What do most people do? The equivalent of leaving the front door key taped to the outside of the front door with a note that says "this is the key to this door". 99% of people using computers who actually give any thought to security (which is probably about 1% of people using computers) fall into one of 2 classes: 1) Computer break-ins are a myth. No one is going to break into your computer. 2) No matter what you do, someone is going to break into your computer, so trying to keep them out is a waste of time.
Most people give more thought to disguising the fiberglass "rock" they hide their front door key inside than they do to securing their wifi router or phone. (And using something like that to keep a door key in is a MAJOR security breach.) Just look at how many people run into the store to grab a newspaper with the keys in the ignition, the engine running, and their child strapped into the seat. If you're not worried about having your CHILD stolen, how security conscious are you likely to be? But these same people don't leave their cellphone on the seat when they run into the store. The phone is more important than the child? "Priorities" isn't a word made up to fill a blank spot in the dictionary.
Wifi may or may not drain the battery a lot faster, it depends on how you have your wifi set up. You should have it set to NOT always scan, and you should keep it turned off when you don't need it. Same with Bluetooth, GPS and 3G/4G data. Do you leave your car engine running 24/7? Why leave your phone radios running 24/7? Using apps that turn wifi on when you're in a location with a wifi signal that you can connect to is another waste of battery. (As are "battery saving" apps, most of which just turn off the things you aren't using, which you can do manually without using a battery-eating app to do it.)