Straight "is" surround sound
Basically, Straight is just the original sound source with minimal receiver modification. So if it's receiving 5.1 Dolby Digital from the source, it will output it as 5.1. Note that I specified "Minimal", however it DOES do some processing, basically to adjust the source signal to match your "room settings" and anything else that is modified from an output perspective. So if you have a 7.1 signal, and only 5.1 setup, it'll adjust the signal some to optimize for your speakers. If you have 2 subs, and they have different sound levels, it'll try and adapt accordingly.
"Pure Direct" is as close to unprocessed as possible, basically decoding the source signal and feeding it straight into the amp stages. I don't know that it's 100% that straight, but basically, if you want to hear what the recording producer's pure original mix would sound like in your home theater, that's it. It might be terrible though, given that the programming of the receiver modifies the signal to adapt to your room's harmonic profiles.
So to sum up, the difference between "Straight" and the other fields, is that it's not programmed as a "venue", and not adapted for 5.1 from a stereo signal, etc. But if you feed it 5.1 surround material (from a DVD or BluRay), it's going to output 5.1 surround, for example. If you feed it stereo, it'll output stereo. But it DOES adapt the input to output based on your speaker levels, and offset times, etc.
Pure Direct is the source signal, without sound field modifications of any sort. So like "Straight" but without customized speaker placement handling, etc. If your room stinks in the 3000hz band, you are going to have terrible quality in the 3000hz band. With "Straight" your receiver has been calibrated and is likely boosting the 3000hz region, so it'll sound better "Straight".
SO... for any surround source, Straight is surround sound, without too much processing (basically pure decoded, adapted for your room's acoustics, but not "Vienna Hall" or "Jazz Club" or "Spectacle" sound fields, it's just as close to what the receiver thinks it should sound like on the original mix, modified to fit your room.
So the only time you'll need a "ProLogic" etc sound mode, is if it's 2-channel source that you want to convert to surround, or you just want a different mode that sounds better for you than the simple "source surround decoding".
I often switch to Pure Direct, which is as close as it gets to the source, it just seems better the way it was mixed. I probably use "Straight" nearly as often though, for broadcasts/uverse "Straight" seems better, for BluRay PureDirect seems better. I've stopped using nearly all other modes except for music type inputs.