Why this may be good for Sprint, a few things to consider...
The WiMAX infrastructure is already built and maintenance costs to Clearwire are very low, as long as they dont plan on expanding it, which is fine because they are in all major metros, it should be a relatively low cost to support millions of paying users. Remember, prepaid customers are willing to receive 4G in only major metros as they pay a lower monthly fee, while Sprint customers expect nationwide LTE. Also keep in mind that with Sprint's upgraded 3G speeds (1.6 - 2.5 mbps), those prepaid customers will be a lot happier. WiMAX may not be as fast as LTE but it still offers good speeds when compared to 3G, I have tested WiMAX in many markets and the average speeds are around 6 mbps, maxing out at around 12 mbps.
Another thing to think about is that 4G WiMAX chip prices have come down substantially, allowing Sprint to make this work. Additionally, wholesale / M2M agreements that Sprint has with major utilities, medical, manufacturing, automobile companies. Some of them run on Clearwire's 4G network and unless they want to move them to 3G or LTE, it would be wise for them to keep the WiMAX Network running as this is another revenue stream for the company supporting the Clearwire 4G WiMAX Network.