For the record, it's not that much work to switch from a Windsor block to a Romeo block. 100's have done it (including myself).
Why would anyone want to switch? Because it can be difficult to locate an affordable salvage yard Windsor whereas Romeo's are very common.
Why are Romeo's more common than Windsors? Because Windsor blocks were in the Mustang for only 2 years (1999-2000). For the rest of the run, Mustangs have Romeo blocks. BUT the Romeo block is also used in the Ford Panther car line (Crown Vic, Grand Marquis, and Town Car). Hence the reason why a low cost Romeo salvage motor will be easier and cheaper to source.