When answering this question, assume Japan is busy taking territories in the Pacific and not threatening Cairo, the Middle East or India.
Arguments in favor:
-In a Moscow push scenario, it’s better to have 5 IPC’s a turn in infantry coming out of Moscow than 2 IPC’s a turn in Britain’s hands
-The Allies overall have +3 IPC’s per turn for as long as Russia holds Iraq, which is good for the long game
-The possibility is open to use the units to bolster Cairo’s defense and/or turn Russia into a beast with 4 Italian territories in Africa at +3 each
Arguments against:
-Russia can also profit by just using all 5 units in counterattacks and the final Moscow battle
-If Moscow falls, Germany gets +5 and Britain won’t have income or an IC there
-Britain needs every territory it can get so that it can project power across the board