Hey Billy,
The challenge for us is there are some web behaviours that Marketo can't pick up on (or can't easily pick up on without code).
Examples -- partial form fill outs, interacting with node.js or Ajax elements (where the URL does not refresh), clicks on buttons in certain modal windows, and the list goes on.
We use KissMetrics to track these things, and we want to send these events into the Marketo activity log. Ideally as "virtual pageviews" that mimic web page visits but with details in the page name so we could use in triggers/filters etc.
Just adding to a Marketo list feels a bit unwieldy to me, in part becuase of having to maintain lots of lists and also because list membership is binary (member or not member) whereas these events could repeat multiple times.