Unity mongering and the Hostility clause

I have recently been accepted as a member of the Socialist Party of Great Britain. It is notorious amongst the Left for, among other things, it's hostility clause in its founding constitution:
  That as all political parties are but the expression of class interests, and as the interest of the working class is diametrically opposed to the interests of all sections of the master class, the party seeking working class emancipation must be hostile to every other party.
For a left obsessed with 'unity' this is incomprehensible. Surely, they say it is vital to unite all socialists and the left together and fight on what unites us. Socialists have no problem with this, but what is being suggested is not uniting of all socialists, a project which the Socialist Party has always been committed to, but instead uniting with reformists and vanguardists on a reformist programme.
        For the past 100 years the working class has been politically dominated by two competing ideologies: reformism, which argued that through piecemeal reform capitalism could be transformed into something nicer. And vanguardism, also called Leninism, which claimed that a violent revolution led by a determined band of revolutionaries could institute socialism with the support of only a minority of the working class.
       Both rejected socialism as being 'Impossiblist', not trusting the working class to be able to liberate itself they instead proposed their different brands of possiblism. Both ideologies have been tested and found wanting; the vanguardists ruled over a third of humanity for 70 years and reformism has held governmental office throughout the industrial world for much of the post War period. Despite this capitalism has thrived and prospered, and the possiblists have been exposed as the worst kind of idealists. Indeed, they have given up any real belief in changing the world in favour of being hired as managers of Capitalism.
        The latest cry of unity was launched by the rape denying SWP. Once the largest fish in the rapidly draining pond of the British left, it has fallen on hard times, suffering repeated splits and a haemorrhaging of its cadres. Recently, the swp has latched onto the rise of UKIP as the latest 'fascist' threat, a political move dictated more by the potential financial benefits of union sponsorship and liberal guardian readers donations to their latest front groups; stand up to UKIP (SUTU) and stand up to racism (SUTR) rather than any real belief that the millionaire stockbroker Farage is a new Mussolini or the gaggle of geriatric Tories and free market nutters he leads are about to unleash a corporate state or a wave of Blackshirt violence.
   After the Rochester bye election the SWP made a impassioned plea for left unity. what this means in reality is illustrated by this weeks protest against the grand jury decision not to prosecute the murderer of Michael Brown. When the london Black revolutionaries who called the protest outside the U.S. embassy refused to allow SWP speakers from their front group SUTR to speak, the SWPs Weyman Bennett threatened to'go to war' with the Lbr, to 'dismantle' them and then to name their comrades to the police. He demanded that the LBR not involve itself in any anti racist activity unless it acknowledges the 'established leaders of the movement' i.e. The SWP. Thankfully, the LBR refused to be intimidated.
      If this is what unity looks like, I'll stick with an little honest hostility.