Cennydd Bowles makes a good point about the intangible benefits of user-centred design in “Why aren’t we converting?”:

I do suggest seeing user-centred design as something wider than just a means of optimising a conversion rate. While there may not be a noticeable uplift in any specific metric, the raw material of design is frequently intangible: trust, loyalty, engagement, etc. These things are much harder to measure, but they still make themselves felt indirectly in other metrics: support costs, referral rates, customer retention, and so on. Separating the effect of design from these long-term figures is, of course, pretty much impossible, but the long-term aggregated data makes it clear that the effect is genuine (see Apple, etc).

It’s a real shame that the results of UX can’t always be measured in a direct uplift in revenue and/or conversion metrics. But it shouldn’t be an excuse not to invest in good design, or worse, to resort to dark patterns.