Well, it probably dilutes both your brand and your link equity. In other words —
1. People who click don't see YOURBRAND.COM, they see TinyURL.com.
2. If posted, search engines see a 302 redirect from DomainX to DomainYOU ... and it eventually expires. That's probably not ideal.
Others may also simply not click because the last TinyURL they clicked was an alias for a certain page on somethingawful.com. The plugin creates the links. Its features include —
* Use 301 or 302 redirects to the authoritative permalink.
* Optionally strip leading "www." from URLs to make them even shorter.
* Optionally send emails that put the link in your inbox for easy access.
* Optionally put the tiny URLs at the end of posts so users can easily place it on their clipboards.
0. Install and activate plugin.
1. Set up and configure options as you please.