They are transparent and democratically accountable, which sets them apart from the donations to political parties by big business and rich individuals.
Unions have to ballot their members every decade for the power to have a political fund.
They have to publish full information on political expenditure and answer to their organisation's democratic processes for decisions made.
On top of this individual members can opt out of political contributions, which in any case add up to around £3 per member a year.
Contrast this with corporate donors such as City banks, hedge funds and other big companies that stump up huge sums for the Tories and, as if by coincidence, reap rewards in tax breaks and other benefits.
Some observers suggest that corporate donations should be subject to shareholder consultation.
It would be far more appropriate for an individual secret ballot of a company's entire workforce since it is the workers' labour power that creates the profits from which shareholder dividends and political gifts are sourced.
The mass media is all but silent on the scandal of unaccountable corporate power funding political parties, mainly the Tories, but it works itself up into a self-righteous frenzy over union involvement in political life.
That frenzy is replicated in Parliament where all but a minority of MPs are either hostile to union financing of Labour or embarrassed by it.
This is another argument for O'Grady's other point about the need for "more bus drivers, cleaners, engineers, nurses, teachers, car workers and miners" in Parliament.
At one time, constituency Labour parties would have had no problem with putting such people forward, but the advent of new Labour put paid to the idea that MPs should look like a cross-section of the electorate.
For new Labour, it is far more important that a small metropolitan clique should be over-represented in Parliament by means of bureaucratic centralisation.
The "horny-handed sons of toil," derided by Peter Mandelson 25 years ago and dismissed as no longer necessary to the Labour Party, found themselves, in common with working-class women of toil, supplanted by parachutists whose main qualification was being "one of us."
This new Labour parachute regiment, lacking direct experience of trade unionism, shares generalised parliamentary hostility to the unions and admiration for big business.
That's why it came naturally to Tony Blair to seek sleazy corporate funding in preference to honest, open trade union finance.
As sure as one hand washes the other, Formula 1 supremo Bernie Ecclestone gave a "no-strings" £1 million donation to Labour and Blair postponed a ban on F1 cars carrying lucrative tobacco advertising.
The Tories have traditionally been tied to the tobacco industry, receiving finance and dragging their feet over measures to protect public health, from denying the link between smoking and cancer to recognising the power of advertising.
No-one should be surprised that a party with Kenneth Clarke and Lynton Crosby at a senior level should procrastinate over implementing health campaigners' demand that plain packaging for cigarettes be brought in.
This government service to the tobacco industry will probably result in lives being lost, but there will be no media outcry over corporate finance of political parties similar to the hysteria over union donations.