The idea has always been stupid, and is nearly dead anyway but that's no reason not to give it another kick.
The cry is that if we vote UKIP we will get a Labour government which is more pro EU and thus somehow worse than the Tories.
It's quite possibly true in certain seats that UKIP will split "the right" and allow in a Labour or Lib Dem candidate who is wrong about even more things than the Tory candidate who might have otherwise got in, but it's hardly the point.
There isn't some spectrum of Europhilia with UKIP at one end and the Liberal Democrats at the other. It's a simple binary choice whether we are in or out of the European Union.
You see the Tories, perhaps more than any other party have misrepresented the EU from the very beginning. This is a 6 decade old project to create a country called Europe through political and economic union. This isn't really a secret as such. It's set out in the 1957 Treaty of Rome which Edward Heath signed in the early 1970s. But to hear the howls from Tory "sceptics" you would think it had never been mentioned before.
Their position is more like Republicans in the US opposing federal government programmes. Well they might, but that doesn't make them anti United States. Only actually being espousing secession would make that so.
The idea that the Tories are "more Eurosceptic" than Labour is nonsense. They may be less in favour of centralised European government (although they may not be too when it suits them) but that doesn't make them opposed to the UK being a member any more than the South Carolina legislature opposing Obamacare amounts to that state seceding from the USA.
For those of us who think the UK should not be part of the European project at all the exact break down of powers beneath the European government (which is exactly what it is, by any measure) is immaterial, making the choice between pro EU parties irrelevant, and the "vote UKIP get Labour" cry with it.
The simple choice for secessionists is vote UKIP and either get UKIP or not. If not, then through ignorance or will the public have chosen to be part of this project.