Looks like this is a temporary arrangement. People of WB are angry at AITMC right now. So for the next two and a half years, this arrangement will cool down the voters. By the time the next Lok Sabha elections come, this group may have control over AITMC.
This is just a pure guess.
Rebel TMC MPs met Lok Sabha Speaker Om Birla at his residence, in Delhi today
After meeting him, Rebel TMC MP Kakoli Ghosh Dastidar said, "We, the twenty MPs elected from the AITC, met the Speaker and submitted a letter requesting to sit separately; these twenty MPs constitute more than two-thirds of our total strength. We are merging with the Nationalist Citizens Party. Moving forward, we will work for the nation and collaborate with the NDA under the leadership of the Prime Minister."