Peak Detectors are included on the device to provide ‘fast attack’ of the data slicer threshold. The midpoint of the slicer can be an RC average of the raw base-band signal or you can average the first positive peak with the noise level allowing for a faster analog midpoint (slicer threshold) generation.
Not using the positive peak detector typically does cause loss of the initial preamble bits in the data packet. By using the peak detectors, the baseband can usually decode all the bits after the first one received.
With regards to the decoding of a weak signal there may be some impact with not using the peak detector but that would usually not be detectable when looking a continuous transmission. The Peak Detectors are most useful for decoding the very early bits in a received data packet.