The sim card is needed to access Sprint's LTE network, since the LTE standard requires sim cards (to the best of my understanding; somebody please correct me if I'm wrong).
I think the phone can be unlocked and used on a GSM carrier, though I haven't tried that myself. According to Samsung's spec sheet, the Neo supports: LTE: Band 25 (1900 MHz); CDMA 1x/EVDO Rev.A: 800/850/1900MHz; HSPA+/UMTS: 1900/2100MHz; GSM: 850/900/1800/1900MHz (emphasis added), so you should be able to get it working on any GSM carrier that uses those bands. I'm not sure how well Sprint handles unlock requests, though.