Its a gamble parties, have to take. They may not get enough seats if they ally before (loosing votebanks) and sometimes alliances also mean you stand in less number of seats so though a chance of winning more seats as a combine is there, as an individual party you might end up getting lesser than intended. Also you allow a party with almost no presence to enter (Of course assuming the other party would ask to contest in a few seats)
Once having taken the no alliance stand, they need to maintain till the voting is over...