The Monte Carlo simulator is a very simple and weak heuristic by itself. In UCT go players it's inseparable from the tree-traversal/expansion algorithm, which uses data propagated up the tree from the MC-tested leaf nodes to decide which move sequence to explore next.

When it's expanding the search tree, every node expansion is arrived at "intelligently". It starts at the root node, and at each branching, asking itself: which following move is the best mixture of good (based on MC games I've played on the leaf nodes of its children) and unknown (ie I haven't explored that branch very much yet). It does that at every node until the leaf, where it expands a node with the MC-simulator (and then propagates the result up the tree). So unlike some other tree-search algorithms, it isn't cobbled by having to expand all paths to a certain depth before exploring a particular sequence deeper. The result literally looks like a tree: some branches are really tall, maybe 18 moves deep, while others are really short. Even though the "heuristic" is MC, I don't like calling it brute force, because it uses the results of those MC games so well.