Simply no.
First of all, PLII supports 5.1 channel matrixed sound. You seem to be confusing it with PLI, which only had stereo and a center channel.
Second, encoding to PL2 is *not* just ripping the track as stereo. It involved decoding all 6 channels and applying phasing to the surround channels so they can be merged with the stereo channels.
More specifically, for those who are bored/geek enough to take interest... ProLogic II is not technically a format. It is an encoding/decoding algorithm. The format in question is, often, Dolby Surround analog. More recent programs, specifically games, have been encoded in Dolby ProLogic II or IIx specifically... but all movies in an analog Dolby surround format are generally Dolby Surround analog to begin with. Dolby Digital soundtracks contain a Dolby Surround analog matrix embedded in the L-R channels for home systems that are ProLogic I/II only.
In the transcoding process, a Dolby Digital AC-3 5.1-channel bitstream is demultiplexed into five discrete full range channels, and one LFE channel. These are L,C,R,SurL,SurR, and LFE.
While Dolby ProLogic II intelligently places center dialogue and LFE by way of filters, the SurL and SurR channels are stereo matrixed into the front L and R channels by way of shifting their signals 90 degrees out of phase with the L and R.
The ProLogic II decoder samples the audio and extracts the portion of the signal at 90 degrees off axis from phase and sends these to the SurL and SurR channels. If the source was Dolby Surround analog, and only one mono surround channel existed, then the SurL and SurR output will be 2-speaker mono. If the source was Dolby Digital, however, Dolby Surround and Dolby Digital encoding hardware/software matrixes the SurL to the L channel, and the SurR to the R channel.. .so the demuxed output is identical to the 5.1 Dolby Digital surround.
A bandpass filter sends frequencies below 120Hz to the LFE. Another filter removes dialogue by... if I recall correctly, a bandpass filter that covers the range of normal human dialogue, particularly if it's centered in the stereophonic image of the playback.
The LFE is not tricky, because if the source was a Dolby Digital soundtrack to begin with, and this will surprise some audio snobs who insist that LFE crossovers be set to 80Hz... Dolby Digital licensed encoding hardware/software applies a bandpass filter at 120Hz to remove all frequencies below 120Hz from the mains and encode them in the LFE channel only. This is by default, but on occasion engineers can set it manually... THX engineers tend to set the LFE low-pass at 80Hz. This is hard encoded in the mastering stage, not applied by the Dolby Digital processor/decoder. Thus, the processor doesn't have to do any "guessing" as to what to send to the LFE on your 5.1 surround system, the cutoff is in the bitstream of the source. Now, this is assuming the source was a Dolby Digital soundtrack. But if the source was a Dolby Surround analog soundtrack, no LFE channel actually existed and the bandpass is applied across the board... i.e. across all full-range channels, and the results sent to the LFE output.
So what does all this mean... It means that the proper Dolby Digital or ProLogic II decoding hardware can do a very good job of discretely extracting 5.1 channels of surround from a Dolby Surround analog mix.
Granted, it will not always sound identical to a Dolby Digital mix, but that depends on more than just one being digital vs. one being analog. It depends more so on how discrete the channels were to begin with (I've heard some DD mixes worse than some Dolby Surround mixes), the quality of the overall mastering of the source (garbage in, garbage out), the quality of the decoder (and whether or not it is a LICENSED decoder), the quality of the speakers, etc.
Keep in mind that Dolby Digital is also a multiplexed format... there aren't five separate bitstreams in an AC-3 file... but digital demultiplexing (at a low cost factor) can be more discerning because frequencies in each channel are reconstructed from packets of data instead of complex frequencies.
HOWEVER, the quality of analog multiplexing can surpass digital multiplexing... but it depends on the quality of the hardware and software in use. Vinyl doesn't win over CD's, though, because at low cost, digital encoding is more efficient at error-free reproduction -- there digital wins out. Vinyl is a very, very dirt cheap storage and reproduction medium. It takes tremendously expensive technology to surpass digital multiplexing in the analog world... and I don't mean a $20,000 turntable. I mean entirely different technology like Dense Wavelength Division Multiplexing (DWDM) which can combine 80-100 wavelengths of light into one waveform that can transmit more than the per second traffic of the entire internet in 1997.