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My answer to your question is maybe not, but don't make any quick simple judgments. First, SDC isn't "bought" it comes with the license. (you'd have to get hardware for it though).
Second, what you are trying to do isn't entirely clear. Referrers to your site can be tabulated by any analytics tool. Marketing analysis means .... the contents of the forms people fill out? That can be done by any analytics tool; the problem is in the coding of the form. Marketing analysis means collecting details about browser settings? Again, you can write page code that collects it without SDC.
I think you need somebody who's really familiar with analytics and WebTrends to figure out whether the person is giving you smart advice about SDC. You can do a lot with SDC that you can't do with logs - for example, put your tag on affiliates' sites so you can continue to collect data even if a visitor moves to another site. SDC also has an extremely smart cookie setup, better than anything I've seen, that has a ton of advantages over regular (log file) persistent cookies or other analytics programs' cookies. Somebody familiar with your situation might be recognizing that you need this.
The SDC tag will also simplify things for you because it scoops up anything you can get into a <meta> in the <head> of your page, if you mark those <metas> properly. In some cases, this alone is worth the price. You can collect all kinds of dynamic page characteristics and stuff them into the <head> without having to add parameters to the page's URL that will screw up your SEO.
But all of this needs to be done by somebody who knows what they're doing. SDC is actually incredibly easy to implement at a basic-but-sophisticated level (their "tag builder" tool on their site allows you to make a pretty sophisticated tag in a few steps) but there are subtleties that, if you site needs them, should be applied by somebody who understands the whole situation.
Either the person who recommended this to you doesn't understand analytics very well and is taking the easy route, or they do understand analytics and your site very well and they're giving you smart advice. Get them to explain why they recommend this. (Plenty of people get all they need out of log file analysis!)
Since you already have the software and server logs, what is it you're not getting that this person things SDC will give you?
There's a site called the Webtrends Outsider that has a lot of tag tweaks and other stuff presented more clearly (IMHO) than the documentation does.
Unfortunately, from what I've been told, the biggest roadblock to merging the data has been risks associated with changing the weblogs. Therefore, I was confused when a lengthy project was proposed to rollout SDC and tag our hundreds of apps/pages. It seems to me that addressing the web log issue would answer 90% of what the business needs to know. Plus we've already established via odbc a connection to the WebTrends data so I'm not entirely convinced that a lengthy SDC project is needed.
Thanks again as this is all good stuff! From a service offering though I'm trying to convince people to keep it simple as a first step!