The interest of Wales is much more important than Plaid's partisan advantages in deciding whether Plaid should go Red-Green or Rainbow.
The most important issue raised regarding the advantage of a Labour-Plaid coalition is ensuring that the clauses in the Government of Wales Act 2006 that promise Scottish type powers to the Assembly after 2011 are activated and activated with Labour support. The Rainbow doesn’t have the Assembly numbers needed to activate those clauses. If a Rainbow call for a referendum succeeded with the support of a few Labour rebels, but the majority in the Labour Party campaigned for a No vote, the referendum would be lost anyway.
However I have a serious doubt about this issue. A doubt about commitment.
If Labour is only willing to support a referendum on extended powers in return for a coalition, but will oppose extended powers without a coalition how committed is the Labour Party to extended powers?
I remember the 1979 referendum, where Labour officially supported Devolution (as did the Liberals) because Plaid, the SNP and the Liberals kept a Labour Government in power in return for the promised referendum with Official Labour Party Support. The only party campaigning with enthusiasm for the official Labour Party policy in Wales, was Plaid. Members of the Labour party were campaigning with gusto for a no vote against their party's policy and without censure.
If Labour is committed to extended powers then Labour should support a call for those powers from a Rainbow Government, not just as a sop for a Red-Green government. Sop support will not win the referendum. Sop support from Labour today will be as worthless as sop support was in 1979.
The only thing that would make me support a Red-Green coalition would be a guarantee that the Government of Wales Act 2006 will be amended to give Full Law Making powers to the Assembly without the need for a referendum, because I remember 1979 and I don't trust Labour on referendum deals.