Well... You HAVE to add the domain to bluehost somehow.
There are three things going on.
1) DNS - this tells the world "This domain lives at this IP"
2) Your hosting - When the world knows to go to that IP (i.e. your host), your host kinda has to know "Oh hey, this domain lives here! no problem." Otherwise people going point domains anywhere and mikasucks.com would show up as a real URL pointing to my website, and it'd be rife with abuse. Even if someone used my nameservers (ns1.ipstenu.org etc), the domain wouldn't work becuase I didn't confirm it by adding it to my hosting.
You absolutely must tell your host that the domain you're mapping really is yours. The common way to do this is by parking a domain or using an add on. Which ever you see fit.
3) WordPress. The plugin does that bit.
I would rather not add every new domain name to my parked domains.
Sorry but you have to. Unless you know of another way to add the domain to your account and legitimatize it. Parking the domain adds it to the nameservers on BlueHost and allows it to work.