The battery is perhaps the most important electrical component on the car, and due to its design and nature, is perhaps one of the most troublesome. As mentioned above, it depends on a few variables. Battery tenders are good for those that have vehicles that sit for long extended periods of time. If you're taking it on a drive a few times a week, you should be fine. If it is going to sit, the battery tenders can help prolong the battery life. I have a vehicle that is currently sitting for a few weeks at a time. I don't have any issues starting it right up, but recently purchased a maintainer for ease of mind.
We do have some info in our tech article and in the comments below the figures (read paragraph above figure 1):
Porsche Boxster Battery Disconnect Switch / Battery Buddy Installation - 986 / 987 (1997-08) - Pelican Parts Technical Article
The tender I use is this one:
Porsche Boxster (1997-2004) - Miscellaneous - Page 10