Isn't the rect pulse filter implicit at the transmitter?
In most waveforms for data communication we do NOT want to transmit a rect for each pulse given the excessive bandwidth required (since a rect is a Sinc function in frequency which is relatively very wide-band). This is the motivation for pulse shaping (such as the commonly implemented root-raised cosine pulse shape): to limit the transmitted spectrum and improve the overall spectral efficiency of the waveform.