Redirecting external links in this day and age is a bit dated and to be honest it's best to avoid. While there is no evidence supporting that it does harm SEO its always best to avoid unconventional ways, you simple can't predict if Google or not will interment this as a slight slap at a future date.
Google has said a few times that they prefer transparent external links since this helps users estiblish where they are going before they leave. I know that redirecting using the ?="url" still allows people to see the end result it also takes a few seconds of thought and to be honest I don't think that's what Google wants - Sadly Google does what it wants and holds us designers/webmasters at there mercy and this is why I advise anyone to avoid redirects, cloaking, javascripts of any short. There is no reason why to use these types of links as there are legit and friendly ways of doing this which I will get on to.
The method you should think about
Most people want to use the redirect method because they believe it doesn't leak link juice. This is non-sense. Google can see it redirects and regardless of if its nofollow or follow still juice gets leaked. Now the problem with any links are they revalant to the page content on the page? it's not a matter of leaking juice, or anything of the sort as I've explained this is unavoidable unless your using blackhat cloaking.
Now the problem is people commenting, adding links which have no revalancy between the link and your page, in effect this can harm authorithy. Now the work around to this problem is simple, 2 methods.
Method 1) Administrate your pages and remove peoples URLS that is not helpful to your site, after all your after peoples comments and not their links.
Alternatively if you want people linking then this will help
Method 2) HTML5 - Still not many people are learning about the benefits of HTML5, with HTML5 it allows us to link out without harming the content of the page thanks to a call element called ASIDE which I'll give you an example of how this works.
Example of HTML5 With Links that are useful for users and about the article:
<article>
<header>
<hgroup>
<h1>How to Use HTML5</h1>
<h2>A beginners guide to HTML5</h2>
</hgroup>
</header>
<p>I am the article about HTML5 Website Design</p>
<aside>
<ul>
<li><a href="#" title="">USEFUL HTML5 LINK</a></li>
<li><a href="#" title="">USEFUL HTML5 LINK</a></li>
<li><a href="#" title="">USEFUL HTML5 LINK</a></li>
<li><a href="#" title="">USEFUL HTML5 LINK</a></li>
</ul>
</aside>
<footer>
<p>You can use the footer to end the article with publish date, author</p>
</footer>
</article> <!-- The Article has ended now you can links that you are concerned about
<aside>
<ul>
<li><a href="#" rel="nofollow" title="">NOT USEFUL LINK</a></li>
<li><a href="#" rel="nofollow" title="">NOT USEFUL LINK</a></li>
<li><a href="#" rel="nofollow" title="">NOT USEFUL LINK</a></li>
<li><a href="#" rel="nofollow" title="">NOT USEFUL LINK</a></li>
</ul>
</aside>
Right, so you can see the above if you notice I have 2 ASIDE elements one within the article and one without. Basically the one in the article will add value to the page while the ones outside are nofollow and since the aside is not contained within the article Google will see this as NOT RELEVANT links meaning that your page authority means unaffected.
Additional Notes:
If your redirecting for monitoring of people exiting pages then I suggest you just use a GOOD Stats monitor ;)