DFT Polyphase Filter bank
I guess you are referring to an implementation like the one shown here. If you leave out the DFT/IDFT blocks, then you don't have the modulation step which is necessary for actually implementing a filter bank, and you will not be able to do appropriate sub-band processing between the analysis and synthesis stages.

When you test your analysis and synthesis stages without sub-band processing in between you will obviously get the same result if you leave out the DFT/IDFT blocks. Analysis and synthesis together are then a pure delay and you might as well replace everything by a pure delay, but this defeats the purpose of the filter bank which is to split the input signal into sub-bands for further processing, and then to reconstruct the original signal from these separate sub-bands.