I would like to read those reasons if you have time. Are there any issues catching the taxis from beach-to-beach and back to ST? Remember, we will have seven people.
Although I am over 60, I still do pretty well on hikes. We usually took one or more hikes each time we went to Hawaii (including part of the Kalalau Trail in 2008). But, it sounds like maybe the beaches are the big attraction on SJ. May not be worth the time a hike takes. They can chew up a good part of a day.
Rain would be what would concern me. Cannot control the weather. How do the turns on SJ compare with the Hana Highway on Maui (if you have ever driven it)? I've driven that a few times. All the turns are tiring, but otherwise not a big deal. For seven people, we would need something like a Jeep Commander if we wanted the height plus 4WD. They are very expensive to rent from what I have seen.
I posted my reasons in post in this thread. Mainly - the hassle factor - and there are transportation options for STJ north-side beaches because of the use of taxi buses that can easily take 7 passengers - and with that many people - you can find a driver that will take care of you (and likely cut a deal).
Ask this question (explaining the situation) on the VINOW BBS - and you will likely get useful advice on this.
http://www.vinow.com/wwwtalk/list.php?4
You may need to register, but it is free.
Ferry schedule and taxi rates are also listed on VINOW - as well as Cruise Ship schedules (important...)
Check out the video link that someone posted - you can see the taxi-buses on the left as the jeep cam leaves beach parking lot.
The Hana Highway is a 'highway' compared to STJ roads - the Max speed limit on STJ is 20mph - STJ is only ~7 miles long...
Also - try and not compare STJ/STT to Hawaii - like apples and oranges - both have their pros/cons (we like both, but for different reasons). If HI moves slow - USVI moves backwards.
Like the Willie Nelson song... 'Still is still moving to me...'
It pretty much rains every day on STT/STJ - a 20% chance of rain (typical for USVI) means that is you stand in one spot - you will get rained upon 20% of the time. Not that it is a 20% chance it may rain - it will rain. It is not only the turns - it is the steepness and the narrowness.
I strongly advise not to rent a passenger van that holds 7 people for just 1 day - not worth the hassle (IMO) - especially if you are unfamiliar with getting around on the islands. Taxi-ferry-taxi - there and back - easy...
the northside beaches are not far from Cruz Bay - and they are serviced heavily by taxi-buses.
Remember: No Worries. Go Slow. Stay Left.
The island people are very different - and can be very gruff. Do not request anything before introductions (Good Morning/Afternnon/Evening) and discussing the weather, etc. first - try and make them smile (I make it a game) - which they can be quick to do when not treated as underlings. It may be paradise to you - but it is home to them - and you are nothing but another visitor making yet another demand.
We will be there in 8 days - woohoo!
I have written trip reports (the last 3 years) that are posted on the Starwood forum (with photos and videos) - to get one viewpoint of vacationing on STJ.
Enjoy.