Changing the absorption rates can have a profound effect on the subjective feel of the drug. With routes like insufflation and rectal (and of course IV and IM) the peak plasma levels of the drug are much higher (for a shorter amount of time) - ie there is more drug in your blood and subsequently hitting your brain at once than with oral. This effects the comeup and the feel of the peak. For some drugs (like 2CB), this almost entirely chops off the comeup, it is a rocket ride that lasts about 5-10 mins rather than 20-40 mins.
I'm fully convinced that oral is superior for a few drugs that I previously thought rectal was superior for. The 2Cs are one group that this applies to. It is better if one has a chance to resituate oneself during the come-up and transition smoothly into the plateau (to get the most benefit out of the experience).
As far as harm reduction goes, it is much safer to take these chemicals orally rather than insufflate or rectal, since they often contain at least 2-3 percent impurity (from the synthesis) and sometimes contain much greater amounts of impurities (from vendors cutting them). There have been cases of toxic impurities being present in some chemicals where taking them orally would not be a problem (MAO enzymatic processes handle the toxins), while insufflation, rectal, or vaporizing would introduce toxins directly to one's bloodstream (w/o the benefits of first pass metabolism).
I think I will still experiment wth rectal, but for drugs such as AMT, DOC, and DOM, which have VERY long plateaus (and long come-ups).