All the other opposition parties are just a waste of time. There is no need for WP or Low to 'give face' to them. They are not credible, poorly organised and poorly marketed, and therefore have no chance of winning. Why should you give way to someone who has zero chance of winning?
At the end of the day, Singapore only needs 3 political parties to represent the entire spectrum of sentiments:
PAP - for conservatives, those who want to keep the status quo, and those who want "stability"
WP - for reform-minded centrists, those who want "stability" but are upset with the current state of affairs and want more fairness in society and in politics
SDP - for liberals, radicals and human rights activists, those who believe that the system is totally rotten and who want plural democracy
All other parties are superfluous - NSP is just a copycat of WP and RP is just a copycat of SDP. As for SPP, they are a party that purely focuses on Potong Pasir municipal affairs, and they will become completely irrelevant once Potong Pasir is drawn into a GRC.