Writing in the Irish Times, Michael McDowell said governments feared a dissident group would fill the void left if PIRA disbanded.
They felt an "inert IRA" would be "the lesser of two evils".
His comments follow news that police in the Republic of Ireland are to re-examine PIRA.
Irish Justice Minister Frances Fitzgerald made the call following a political row over the murder of Kevin McGuigan Snr in Belfast earlier this month.
PSNI Chief Constable George Hamilton said the IRA still exists as an organisation, but not for paramilitary purposes.
He said individual members were involved in the recent killing of Mr McGuigan in Belfast but the PSNI has no information to suggest it was sanctioned at a senior level.
Ms Fitzgerald said there were "no simplistic answers" about the continued existence of the IRA.
Information available to her suggested that it remained on an "exclusively political path", she said.
But the minister said that, given the current police investigation into the murder of Mr McGuigan in Northern Ireland, the Garda Síochána would carry out a fresh assessment.
Sinn Féin TD Brian Stanley said Ms Fitzgerald had "abused her position to make smears of illegality" about his party.
"Sinn Féin does not benefit from any form of criminality," he said.
Mr McDowell served as Irish justice minister when the IRA announced it had stood down in 2005.
In his article, he said that at that stage, the two governments had "a clear political calculus".
"The choice was between an IRA that became an inert, unarmed and withering husk or an open goal opportunity for dissidents to reform an army council as the legitimate heir of the body which had been 'treacherously' wound up," he said.
"Past splits and schisms in the IRA showed only too clearly that the IRA could more easily metastasise rather than wind itself up. That was seen, and I think rightly, as being the greater evil to be avoided.
"The governments took the view that an inert, freeze-dried husk of the IRA was preferable to passing the ideological torch to the dissidents.
"The analogy that was used at the time was that it would become like the 'Old IRA", a harmless grouping. That is what [Sinn Féin president Gerry] Adams warranted would be involved in the IRA 'going away'."
The Irish and British governments allowed the Provisional IRA to continue as an "unarmed and withering husk", a former Irish justice minister has said.