The concept of a "primary" smartphone is a little strange to me. How common is it to have multiple active smartphones? (As opposed to, say, one for testing software, or an old one in the closet)
I do it because:
a.) iPhone OS is better but overall, but the tiny screen is super super annoying for many things (IMO)
b.) this way I have NTT Docomo in addition to Softbank, so I get coverage underground and weird places where one or the other doesn't work
c.) voice dictation is 100x better on Android, so I use the nexus for dictating things
Not only provides sandbox to play in, it has twice provided an acceptable backup during repairs to primary with only a simcard swap.