I was under impression that when you signed up for destinations; you were exchanging weeks for points; the pts they were offering for my late August week at Hilton Head would only get me a few nights at other resorts.
I'm afraid there are more questions that you need to answer, for yourself and others, to help you make an informed decision.
1. From where and when did you purchase your owned week on HHI?
If it was from Marriott directly, you should be able to enroll it for a flat fee without having to purchase additional points. They will want to sell you points on top of enrolling, but you don't have to buy them as a condition of enrolling a week purchased from Marriott.
If you purchased your week on the resale market, it is eligible to be enrolled one of two ways. If you purchased your week prior to June of 2010 (approx.), you are eligible to enroll your week by paying a flat fee (but it will be higher than the flat fee for someone who bought their week directly from Marriott) and you do not have to purchase any additional points. If you purchased your week on the resale market after June of 2010 (approx.), you are eligible to enroll your week if you purchase a minimum of 2,500 points (a likely $25k+ expenditure).
If you enroll your week, you will have the option of converting it to a pre-determined number of Destination Club (DC) points each year and use those points to make reservations of 1 or more nights at a time. How much any given week is worth is based on resort, season, view, etc. (However, I think most owners would agree that there is a 'skim' factor where the DC points conversion value assigned by Marriott to any week is not enough that you could go to your own resort in your owned season and get a one week stay). In that regard, your owned week is more valuable than it's points value.
2. How do you want to use your ownership? If you are happy going to your owned resort for a week at a time, or are fine exchanging through Interval International (II) for week-long stays, the option of converting your week to points may not be something you would use. If you would want to travel for less than a week at a time and/or travel during the 'off-peak' or shoulder season, being able to convert your week to points may be beneficial. (II does offer a 'short-stay' exchange option for stays of 1 to 6 nights, but I don't want to divert the conversation from your question of enrolling or not enrolling by going off into that. Just know there is such an option without going the Marriott DC points route.) Traveling 'off-peak' it is actually possible to stretch the value of a week and get more days of usage from your ownership through points than the 7 nights of holding/using it as a week.
3. Lastly, if you decide that you do want to purchase points you should be able to get the 'enrollment fee' for your week credited toward whatever increment of points you purchase (1,000 points is the minimum) to reduce your price per point. One benefit of having a DC account is the ability to rent points from other owners who have excess. I recommend considering only a minimum purchase of points (if at all) and looking to rent anything more you need beyond that.