No. It still only uses one core. The author is funny about it and has resisted suggestions that it might be a good idea. Some of the users have contributed worked arounds to run multiple copies in different accounts, but it is a PITA hack.
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.
4Q.. you're exactly right. Its for the guy who likes to experiment and tweak things... The unfortunate thing is I have been caught up with optimizing signals, etc and its very addictive because it only takes 30 seconds to get an answer.. so this is probably detrimental to pure trading, but Amibroker for someone starting out learning some basic quant principles is good.. Usually you don't need to deal with the owner for most things, his assistant is usually thorough, and the user community are pretty friendly - Howard Bandy has some useful books out as well on Amibroker and developing systems for it.