Of course UKIP aren't going to win in 2105, and this may put people of voting for them. But the point is that they have been catapaulted into a position of prominence, of being a mainstream party for the first time. While not necessarily going to win, UKIP could have the impact of making a severe dent in the Conservative vote, and perhaps even the votes of all of the three major parties. That could lead to some UKIP seats, or none at all, but it will certainly not be negligible. You don't need to win to be an influence.
The Conservatives in particular are at risk because that party is already divided over Europe. If UKIP can come up with a serious Economic policy they could pull people on the right in over that. Just as possible, then, as a Con-UKIP coalition, or a continued Lib-Con coalition, is the possibility of a Labour Government, not because more people wanted them in power, but because the vote on the right becomes split.
The arithmetic is nothing like simple -- UKIP will not win, but how well they do, or not, could have a major impact on the fortunes of those who can win.