Here's what the opinion polls say when UKIP is in the "Others" section ...
|UKIP in "Others"|
... and here's how it looks when UKIP is prompted alongside the LibLabCon parties ...
That's a whole 5% difference and at the expense of both Labour and the Tories.
These polls are just a snapshot of public opinion and vary by a couple of percent from week to week but they're important because they aren't just a snapshot of public opinion, they influence public opinion as well. When a party does well in opinion polls it gives the public more confidence in them but when a party does badly in opinion polls it perpetuates the "wasted vote" myth (no vote is wasted, whoever you vote for). Success breeds success.
Some of the polling companies - and YouGov in particular - like to use this influence to further their agenda (YouGov's CEO is Baroness Ashtray's husband) and protect their business. Polling companies get paid to predict elections and the last thing they want is an interloper like UKIP upsetting the status quo and making their lives difficult so they do what they can to undermine voter confidence in the party by suppressing voting intention.
Contrast YouGov, who don't have a very good record at predicting UKIP's performance, with relative newcomers Survation, who do. Survation don't have an axe to grind and being a new polling company they're more interested in predicting results rather than influencing them. They've always prompted for UKIP and as a result, they've had a degree of success in predicting UKIP's performance. It's time the other pollsters - YouGov, Opinium, ICM, ComRes - followed suit and concentrated on making predictions that reflect reality instead of trying to make reality fit their predictions.