It would cause an issue if we ever called ```this._plugins.toString()``` in our code.

```
> a = {}
{}
> a["toString"] = 7
7
> a.toString()
TypeError: Property 'toString' of object object is not a function
    at repl:1:4
    at REPLServer.self.eval (repl.js:110:21)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)
    at ReadStream.onkeypress (readline.js:99:10)
    at ReadStream.EventEmitter.emit (events.js:98:17)
    at emitKey (readline.js:1095:12)
```

Worse things would probably happen if they named their plugin something like ```constructor``` or ```hasOwnProperty```. Sure, a name like ```hasOwnProperty``` would be weird. But seems good to be safe. It ain't hurtin' anyone to write it the correct way. :-)