Well, by using delphi your working against the OSS character of ROS, that is, it couldn't be used in the ROS project. There's no OSS Delphi compiler, and there are legal problems with delphi as well (see XPDE). And as last point, afaik Delph isn't crossplatform.
You do not know anything about Delphi. ;-)
It actually is (limited) crossplatform, as there is an implementation for linux named Kylix.
and it may be free (I do not know and do not want to know details), as there is the emerging lazarus project. lazarus is near to finished for basic functions. Lazarus will be fully cross platform.
My program nearly compiled on lazarus. It complained about a missing gcc (i did not investigate this issue further) well, some VCL components are still missing. http://www.lazarus.freepascal.org/
In my opinion, if you say, windows is good, because of it's ease of use, you must say Delphi is good too. so, ReactOS without Delphi in focus is only half it could be.
regards
Jan