As boring as it might be, a strong opposition is necessary vs a stronger government. The Conservatives may tear themselves apart over the referendum but they are very aware of that and are trying to make any internal strife as small as they can (e.g. no official party line, ministers can campaign any way they like, etc.). So long as the Conservative government continues to push through agreeable legislation that the backbenchers support, so long as the party stays more or less united over domestic issues, the opposition is either going to be united or wholly ineffective.

At the same time, the other parties are very keen to continue pushing the Labour Party to be weaker and less united - UKIP want to try and turn a lot of their second places into seats & make longer lasting gains on the white working class vote, the Lib Dems are keen to recover the hordes of voters they lost in the last 6 years back and then even take some of the long time Labour voters who feel that Corbyn is a bad leader and the SNP have already grabbed the majority of past SLabour voters.

Conversely, who of the other 5 parties want to push the Conservative split over the EU? I feel as though among the pro-EU side (Labour, LD, SNP) there is a definite worry that weakening Cameron on the EU is dancing on a razor's edge, especially after the general response to the negotiations. While they'd love to marginalise the Tories they most certainly don't want to leave the EU. And as for UKIP I just don't feel as though they are in a position to push the Conservatives to make the Leave voices stronger and even go for defections

Generally I get the feeling that the tertiary parties don't see a massive risk in pressuring the Labour party over the issues that are dividing them but do feel the same risk in pressuring the Conservative party over the EU.