East Hampton Town’s planned purchase of the development rights on the 35-acre Whitmores landscaping nursery on Long Lane presents a dilemma. On the one hand, the $3.2 million deal would prevent the site’s ever being turned into a housing development. On the other, it does not appear to do much for the town as a whole, provide public access, or assure the land’s return to crop growing. A hearing on the purchase is scheduled for tonight at 6:30 in Town Hall.