A lot of Fremantle apps will work on Mer, since it has the new and updated APIs. Because being active in the community helps the community, which in turn helps you. Because being active in the community gets you rewards like possible future device discount vouchers, summit sponsorship. Because the price of RX-51 might be so low as to entice you to buy one. Because the features of RX-51 might be so mind-blowing you'll spend more money to buy one. Because you don't want to spend all day trolling a forum like an arse? ;-)
For your average application developer, Mer isn't an offshoot.