People like talking bad about ModPlug's IT/XM format hacks, and quite often they are of course right - ModPlug shouldn't extend these formats to add features that will either crash the original program or that simply make the file sound completely different. But is this true for all new features?
I wouldn't say so. One feature where I feel that quite the opposite is true are, surprise, instrument plugins (VSTi)! Why would I say that, given that VSTs are probably the most criticized feature addition? Especially VST instruments, which stay completely silent when played in any other player? Well, that's because those other players simply don't implement one of the most interesting features of Impulse Tracker and Fasttracker 2: MIDI output.
Let me give you an example. Do you hear anything apart from a drum beat when playing this with your favourite IT player? No? You don't hear an organ? Well, sorry to disappoint you, but that's not ModPlug's fault. This file was saved in Impulse Tracker. It plays correctly on Interwave and AWE soundcards, since those are the only two drivers in IT that can have sample playback and MIDI output at the same time. Instrument plugins are just a layer on top of this MIDI mechanism, and instead of having to tell people which synth they have to hook their computer up to, you can simply store a reference to a softsynth and all its parameters in the module file, making this awesome feature of IT and FT2 way more portable. If you compose a song with VST instruments in OpenMPT, you can, in theory, load it into IT, use one of the two MIDI drivers and play it there as well! The additional information that's required to store the VST references won't interfere with IT or FT2, so it's no problem really.
I do agree, though, that effect plugins are slightly more critical than instrument plugins, but keep in mind that there's worse - like IT's AWE32 driver which could enable hardware reverb and chorus, but the settings for those two effects were actually not stored in your IT files. This is just like adding a chorus or reverb plugin to your song, except that with plugins, you can also store the exact settings. I do not necessarily think that it's a good idea to spread such IT files with embedded plugin settings, but the same is of course true about IT files written in Impulse Tracker that make use of AWE32 effects or IT files that make use of Impulse Tracker's MIDI output. And the same holds for XMs, of course.