I think that Tom's point about how groups behave inside a formation like a Socialist Alliance is important. Obviously if members of the different groups making up the SA are so bound by their group's discipline as to prevent them from engaging in debates or action in 'good faith', then the SA will fall apart. The Trotskyist left in the UK (especially the SWP) has a bad track record on intervening 'parasitically' in campaigns - going into them with the perspective of recruiting rather than getting a job done for the working class. The old Socialist Alliance and Respect are two clear examples of this mode of operating. Tom is right to bring up the example of the LCR as something that gives us an idea of an 'antidote' to this culture.
But I think that Tom would be wrong to pose the question in terms of "if a group is serious about the SA then it will dissolve itself". It's not a case of EITHER dissolution of all groups OR an unstable sectarian lash up between rigid parties. The LCR shows us a middle ground. The LCR was, for many years and right up until it dissolved itself, made up of different "platforms". These platforms had a right to argue their differences publicly, produce their own bulletins and statements for consumption inside and outside the LCR, and organise their own educationals and conferences. But at every congress they would dissolve and re-form, and membership would fluctuate. Generally a platform inside the LCR would be constituted on the basis of a big difference from the majority line on one or several questions - and these differences would be thrashed out in pre-conference discussions and correspondence. At the conference they would be officially constituted as "Platform A, B, or C" or "Platform 1, 2, or 3". Of course, membership of these different platforms would often carry over year-on-year, and some more permanent factions existed, like the SWP section SPEB, or Débat Militant, who would join whichever platform most resembled their own ideas... however sometimes the membership of one of these factions would be split across more than one platform... It's perhaps a bit of a confusing picture I'm painting here, but the point is this: these platforms, in conditions of free, open and honest debate within a flourishing, active and democratic League, became fluid expressions of changing ideas within the LCR, rather than sectarian, self-building mini-parties.
Roughly the same arrangement has been carried over into the much more heterogeneous NPA, and some other Trotskyist groups have entered the NPA as factions in their own right (like the CWI section, I believe, and the old LO Fraction, Étincelle). Also, the LCR didn't 'dissolve' itself totally, because you can't just stop a whole body of same-thinking people from agreeing and acting together just by dictat. Instead, the LCR is in the process of constituting one or several currents inside the NPA.
I'm not arguing that this approach is something that can be transplanted straight away into a UK Socialist Alliance tomorrow. You can't just wave a magic wand and make parties whose previous basic mode of functioning was telling disgraceful lies suddenly start being open and honest with each other. In France, it was very difficult as well, and when the Débat Militant group joined the LCR in the early 2000s, they were subject to a lot of suspicion and hostility as perceived "entryists". But what it shows is that if a political formation is dedicated to honesty and democracy, if it allows internal dissenters not only to speak but also to organise, and if all its members can be convinced that it is worthwhile in and of itself and not just as a recruiting tool, then different political factions can co-exist within that organisation - and have their own publications, conferences and internal discipline! - without tearing it apart. It can be done: and the factions don't have to be artificially dissolved - but under these conditions their 'walls' become porous and their membership becomes based on ideological agreement, not sect loyalty.
I think that we would need to go into a Socialist Alliance with the perspective of wanting to create a multi-platform organisation like the LCR. But obviously we should also, as Jason and Tom point out, want this to be much more than a stitching-together of existing Trot organisations: it has to be an activist party that can represent local and national working-class campaigns and industrial struggles.