I've never fully bought that argument.
When deciding what to cancel and what to give a trade to first to see if it has an audience there, they will take into account various factors. One of those factors will be the sales of the monthly, so in theory, if you have a particular favourite you might be able to improve its individual chances by buying the monthly. If that happens a lot though, the relationship between the sales of a comic and the trades will shift so higher monthly sales would be required to guarantee a trade.