It's a big symptom, not a big cause of additional problems (though, IIRC, breaking a big piece of the glacier off will increase the speed that the remaining glacier -- including the parts upstream still on land -- flows out into the ocean, which will result in sea level rise.)

Also, melting sea ice itself increases absorption of solar energy and accelerates global warming slightly. Its not a huge factor, IIRC, but you never want a positive feedback loop that reinforces and undesirable trend.