The simple answer to your question is "yes, there is a risk of that happening". OTOH, there are places in the country where UKIP split the anti-Tory vote (like Cumbria for example). Not all UKIPers are ex-tories. There are ex labourites, and there are an awful lot of people from apolitical backgrounds, making their first moves into politics. And that is very good to see.
There are other responses to the splitting question however. If you follow the arguments of the "a vote for UKIP is essentially a vote for Labour, as it will split the conservative vote" crowd to their logical conclusion, then there will never be any change in the political landscape, ever (unless a fourth party splits the socialist vote at the same time). The problem with this attitude is that you end up voting for candidates and policies that you tolerate rather than agree with. You end up voting AGAINST what you really disagree with, rather than for what you do believe. And that's really rather negative. If the Conservative party, or republican party in the US, has drifted so far from its traditional base, then they are effectively relying for support on simple tribalism, not political or philosophical agreement. That is a very bad state of affairs. In the end, it results in lower membership, less funds, less foot soldiers on the ground, and poorer policies.
So I guess in the end you have to weigh up the possible short term disadvantage of losing a few elections with the long term advantage of being part of an organisation that really does listen to and make sense to its supporters. In the final analysis, people will respond better to such an organisation and will be more likely to support it with their time, gifts and money. The alternative is a lifetime of choosing between the "totally wrong" and the "partly wrong".
Just for the record ,i did go-for-it last night.
Good man! For the record, so did I, and it was an easy decision as only Labour and UKIP contested my constituency. Unfortunately, we lost, but we did very well, considering we came from a standing start. Certainly its going to be brown trouser time at both Labour and Tory HQ's this weekend :)