Why is "Fragmentation" a problem

I keep hearing that fragmentation of android is an issue and Google needs to do x y and/or z to "fix" it. My question is by what metric are people saying it's and issue. I have found blog post saying that android fragmentation is going to kill the platform as far back as January 2010 (right after the Christmas season with Verizon pushing the droid does ads.) Since then Android has done nothing but grow in market share blowing past iOS on phones and smothering windows phone, yet the fragmentation arguments are only getting louder and more frequent. By all accounts Google's plan to open source android and every decision they have made about android seems to be paying off in android's market share growing. So I ask the people why is fragmentation an issue (Also everyone please take a good hard look at Windows market share pie chart and tell me why that's not fragmented.)