Having open boarders is extremely good for an economy, people come here and take jobs that others find undesirable, this increases the amount of money circulating within our economy. The alternative is those jobs are sent overseas, which means that our economy wouldn't see as much of a benefit.
The problem we have is our welfare state is too accessible to those who don't need it so you get people coming from poor EU countries with relatively poorer welfare states to the UK where they know that the government will provide them with a better standard of living. And do you know who funds it? You (providing you pay income tax).
No! That doesn't happen! Habitual Residency Test etc. It's a complete myth that EU migrants claim benefits en masse, it just doesn't happen.