There are two problem with rand():

• first is that it isn't a particularly good uniform random generator. You can see this if you generate a large set of numbers and produce a histogram. You should find you have a near straight line, with numbers in all ranges occurring equally.
• second the repeat period of the rand() isn't as height as some other generators. In otherwords if you try to produce a sufficiently long sequence of random numbers, you eventually reach a point when the sequence is identical to something produced earlier.

There are a couple of alternatives, some of which are on CodeCogs, have a look at this category: http://www.codecogs.com/pages/catgen.php?category=stats/random