Hi I can answer this question quickly and easily.

Disadvantages to developing your site in DNN : None

Advantages to developing your site in DNN : You will be able to create your membership site, give it a nice looking design, implement a paid-subscription model with anything from video sharing to forums to e-commerce.  You'll be able to do all of this quickly and easily without having to write a single line of code - as all of the features I've mentioned are available either as core modules (free) or as third-party modules (cheaper than the cost of a 1 days development effort).  There is a wide choice of skins either for free or for a small cost, given the amount of time it takes to develop and test a new skin out.

For your subscription needs, the DNN Core has built in subscription tools.  Also look at the subscription tools offered at http://www.ventrian.com/ - this is enough for most sites needs.

If you want to get the site up and running quickly and easily, with DNN you should be done by the end of next week.  If you want to write your own, if you're lucky, you might have the technical spec written by the end of next week.  But the end result won't be as flexible, as fast, or have as many features as DNN does out of the box.  You just can't argue with thousands of hours of existing development work provided for you.