[Vanders9]

I think you can get round the problem in various ways, and as variety is a good feature of English and a main reason why it has produced such good literature over the years, it is something to be aimed at!

In the example you gave, I would have said "Whoever wins this raffle will have a nice prize" and in the second example, I would personally have just left the last two words off. Problem solved. However, I am beginning to hear things like that last example a great deal more, and although it is grammatically incorrect, it sounds very natural to me.

[annalex]

I agree that the ungrammatical "they" is a solution in search of a problem. Of course, my examples were artifically compressed into a single sentence. Usually, the speaker drifts from singular to plural and back, so even when no single sentence is ungrammatical or strained, the entire speach is.

although it is grammatically incorrect, it sounds very natural to me.

No. This practice destroys an important grammatical feature. Although I have to admit that would nto be the first: the English did drift into mixing "thou" and "you" a long time ago, and more recently, subjunctive "were" and "was". But neither of these corruptions were salutary either.