The problem is that Marriott points are a bad conversion. They are supposedly worth around 0.8 cpp, and Avios probably around 1.3-1.5 cpm (in USD cents), so if you convert you are essentially buying Avios at 2.4 cent per avio (in US currency). The boosting offer obviously then drives the price down for the 3x boosted, but if you add these up you end up buying Avios at 1.4 US cents per Avio, per my calculations. Not terrible, but different from the regular 1.1 US cent per avio with the boosting and the 10% discount.