If I reformat my hard drive, would I still need to use Eraser?

Im no expert here but if you want to be sure the data is destroyed, then yes you should still use eraser as simply reformatting isnt secure. Better still use Dban instead of doing a reformat and kill 2 birds with one stone.
A low-level format will defeat recovery software like EnCase. If you are in risk of being under investigation (maybe due to illegal activites online... lol :p ) then a low level format will do. Most law-enforcement agencies use EnCase as standard, so you will be fine.

Honestly, people shouldn't be so worried about secure destruction, unless you're a terrorist or something.
I think securely deleting any sensitive data you may have on your pc is simply good practices. Whether you actually need to, for whatever reason, may be another matter. It's really no different then using complex passwords. Though you may not be in any danger of having someone trying to access your accounts and what's still good practices.

One good argument for routinely erasing your data is you may eventually pass your HD on to someone else. What you had on there, whether sensitive or not, is really no elses business.

Whatever the cause. I have no problem with anyone wanting to securely destroy data, I do it myself and I believe more people should practice it and people should be aware that deleting files doesn't mean they're gone. I was just saying is all.