I think the benefit of focussing on hotels.com in particular is that you are building up towards a "free night" (i.e. the average of your spend across 10 nights) while also having access to a wider range of hotels rather than being tied to one brand/programme, which can often means missing out on more interesting/better located options depending on the city (e.g a choice between a tired Hilton/Marriott/Sheraton in a business district just to earn points or a more boutique option in a more interesting location).

YMMV of course and different people want different things from hotels, but I like that balance that hotels.com provides - although in practice I do a mix of booking through them and hotel groups/programmes directly depending on the overall cost/benefits mix.