The reason for Multicharts is a pretty simple language for the logic, user friendly, support for many feeds/brokers and a decent community.
Ami also has a good community, but the program is not as user friendly. It can however be customized more if you're looking to create a certain type of trading environment. But the programmer running the show is a real prick, unfortunately.
Anyway, the two programs aren't really targeting the same types of traders in my opinion. I definitely see Amibroker being the right choice for a certain type of traders, maybe particularly with a techy type background.