Well, I agree to disagree then!
IMHO porting software to another platform can give you something that maybe is more valuable than simple $ profits: influence. influence on program desing, on customer experience, on digital lifestyle trends. All these are closely connected to profits again: interest in Apple software and hardware, exposure, trendsetting, etc.
On porting the engine vs. tweaking the code, I think it's a quality/quantity problem: porting the engine might be more difficult, but tweaking the code of the entire iTMS might require more time because of the sheer bulk of the thing. However, IMHO porting the engine has several advantages above tweaking the code, because it will enable Apple to easily port Safari to windows, which I deem a good strategical move for the reasons above.
Giving windows users free apps to play with (iTunes, Safari), might in the future result in interest in the iLife package, if and when it will be ported to windows. I do not exclude that iLife might be made only commercially available to windows users at first (not as a 3/4 free download).
It is true that Apple in acertain sense is a hardware company, because they make most profits there, but still, they do nevertheless need exposure, publicity, influence, etc. Moreover, they do have a software side, wich is not entirely irrelevant, because many Mac users advocate the quality of the OS and software as main points for choosing the Mac. Making inroads in windows territory with software, helps generate more hardware sales.