If you are not planning to breed then the only issue I can forsee is if you wanted to grade the foal. The BEF are proposing changes to the Futurity scheme which will exclude youngsters who are sired by un-graded stallions. What do you want to do with the foal?
Grading as mentioned above, also if you wanted to show in breed or part-bred classes. If you don't want to do either and like the horse as it stands, then I wouldn't be terribly put off. What is the stallion like, and why isn't it registered? If it is because it looks like it has 3 legs and is riddled with inheritable conditions then I might think twice though!
Hopefully going to be used low level showing and eventually break as an event/competition horse.
I wasnt sure, just thought I would check as Im not planning on breeding. To be frank I am on the wagon of there being too many young horses being produced willy nilly without any thought on breeding/conformation etc.. hence we are becoming over run with horses.