Many contemporary artists, chief among them Damian Hirst, use assistants to create their work. In terms of the value of his work, the use of assistants has not diminished it. In fact, the diamond-encrusted skull which Hirst conceptualized (but did not make) sold for its asking price of 50 million British pounds.
Some of Hirsts' assistants are named and artists in their own right. They do get some credit but are not generally discussed and are never put in the same category as their boss.
A quote from an article discussing Hirst says:
"Fifty million pounds might seem like a ridiculous amount of money for diamonds arranged on a platinum skull, but it's all about the uniqueness of the idea and the fact that there will always be only one piece like it. Mark my words, it will be sold."
Hirst is not the first to employ these methods--the great masters of old used many assistants to create their famous paintings. The value of those works remains extremely high, and it is not considered a 'black mark' that assistants were used.