Basically we think that in the conditions of modern day capitalism a serious attack on our class can only be resisted by going "outside and against" the unions. Certainly an offensive against the system and the possibility of revolution emerging from this would not be possible within the framework of the trade unions (or any other union form).
How this has happened to some extent in the past and might happen again in the future is however a practical question. It is not achieved by small revolutionary groups, still less individuals, taking up moral positions towards existing unions and their activities. What we do as revolutionaries is guided by our historical understanding of the role of unions, but it depends on the particular situation we are addressing.
It can vary most obviously according to the general level and intensity of class struggle in the local and world context. It could also vary to some extent depending on the nature of a particular struggle, the unions involved, the workplace and the local history of struggle and local culture.
We have to judge what we do in terms of how it will help (if at all?) move the class struggle forward and increase the confidence, solidarity and autonomy of our class.
For Subversion then - a small group operating in Britain in 1998 - we do not oppose individual membership of trade unions. Generally speaking we do oppose taking up official positions (even unpaid ones), but some members have occasionally in the past temporarily become shop stewards where they thought, on balance, this was useful to advance a particular dispute (mainly because this was the easiest way to get meetings organised in a situation where the level of struggle was low).
The point of this long preamble is to stress that the above position isn't so much a matter of principle as of practice and tactics. We can imagine, for instance that in a country with a totalitarian political regime and compulsory membership of a government union that individual revolutionaries would belong to that union out of necessity. On the other hand in some countries now (e.g. France and Italy) where union membership is low and divided between numerous different political and religiously affiliated unions, but where there is a history of independent struggle against both unionised and non-unionised workers, revolutionaries might refuse to join any union.
The point is to move struggle beyond the framework of the unions and unionism in whatever practical way we can. Hope that's clear?
On your second question about the "capitalist left"...Capitalism involves the domination of society by commodity production (the market, buying and selling, money, etc.) based on the extraction of surplus value by a minority which owns and controls (either directly or indirectly) the means of production and distribution.
That said, there are many ways of organising and managing capitalism from individual ownership, through partnerships, joint stock companies, trusts, local and national state enterprises, multinational companies and co-operatives, etc. There are also various forms of political organisation of the state depending on historical development and local conditions. Revolutionaries oppose capitalism whatever form it takes.
Generally the "left" has favoured various forms of state ownership and control sometimes in association with 'co-operative' ventures. They have not opposed - in practice - commodity production, wage labour and the state. More often than not in times of intense class struggle the 'left' has proved itself to be the saviour of capitalism not its gravedigger and this despite the "good" intentions of some individuals within it! Similarly some anarchists whilst opposing the "state" (at least in theory) still promote a form of decentralised market economy - they are really just radical liberals not revolutionaries. For all these reasons revolutionaries refuse to join "fronts" with organisations of the capitalist left. This doesn't of course mean that we can't work with individual 'leftists' in various practical ways for limited objectives - as we have for instance in anti-JSA groups, dockers support groups, some environmental groups etc. and in particular workplace or other struggles.