Baby sign language, when done properly, definitely does not make children late talkers or hinders language and speech development - research has shown that it does quite the opposite. It is a fantastic tool to aid the development of language and communication. The important factor is how you use the oral langauge in combination with the signing.
For example - don't just sign 'more' to your child, but say to them 'Do you want some more? More?' whist signing 'more' when you say it. When your child signs 'more' back to you, say to them 'more!' you said you want more', signing 'more' again each time you say it.
Obviously if you just signed and didn't speak with your child, they would learn to sign and not speak, but if you use the two in combination, they will learn to use sign as a communication tool to aid in the development of speech and language. Many child care centres use MAKATON, which is a way of using AUSLAN (Australian sign language) to aid in the development of langauge, particularly with children with special needs, but it can be helpful for communication at any age. Have fun with it!