You don't have to use an instrumental. But you can use one if you want.
Cutting your entry from multiple recordings is not allowed because it affects the performance. If you can't do it well in one take you're effectively gaining a positive effect from mixing your takes into one file and that's just the same as any other kind of mixing.
Hiiro Sakaki also gave some good points.
The question was already asked before and got a clear answer.
/edit: If it features the original singer then HOW is it an instrumental? Instrumental = instruments and no voice. wiki-link
Cutting your entry from multiple recordings is not allowed because it affects the performance. If you can't do it well in one take you're effectively gaining a positive effect from mixing your takes into one file and that's just the same as any other kind of mixing.
Hiiro Sakaki also gave some good points.
The question was already asked before and got a clear answer.
/edit: If it features the original singer then HOW is it an instrumental? Instrumental = instruments and no voice. wiki-link