Exclusive content is more likely to get new customers, which is what's really important for MS. If MS lost exclusives and so reasons t buy an XB360, the fact the machine's reliable isn't enough to sell it! After landing the customers, they can sort them out however afterwards. By then they'll be able to point their large install base to devs and hey presto.
Putting it another way, if the XB360 had launched at a higher price because of a better quality build, would it sell even as well, let alone better? Build quality is something people tend not to care about until after the purchase when it starts affecting them. If XB360 gains a reputation for breaking down amongst the general public, which I don't know that it has yet, it's still the cheapest solution for HD gaming and with exclusives people want. It still has key selling points that differentiate it and will see people 'taking their chances' and buy it despite the chance of failure.
IMO, the main reason for sorting out failures is to save cash on servicing! This must be costing MS a bundle. As came up in prior threads, those who doubted a 20% failure rate said it cost too damned much and MS would have fixed it if it was that high. Well, seems like 20% isn't too high and MS are losing that much money. Maybe they've been unwilling to redesign the system before 65nm components are out, and perhaps these have been delayed and delayed?