<br>I see this discussion going back and forth and many people repeating what they said before only slightly more extensively, or with slightly more examples. This has turned into a textbook example of basic logic. A random case from a random person (anonymized , albeit rather too simplistically, because it is no more than an example) is this one:
<br><br><div><span class="gmail_quote"></span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Let me explain why I think that LateX and plain tex cannot be
<br>combined and why I use \@ifundefined as an example.<br><br>The definition of \@ifundefined is<br><br> \expandafter \ifx \csname #1\endcsname \relax \expandafter<br> \@firstoftwo \else \expandafter \@secondoftwo \fi
<br><br>Hence, if someone says \@ifundefined{foo}{}{} and tries later<br>\ifx\foo\undefined this will always succeed because \@ifundefined sets<br>undefined cotrol sequences to \relax.<br><br>You cannot safely use \ifx in LaTeX.
</blockquote><div> <br>One example is not going to prove a general point ever. Nor are two, ten, or 10000 examples. Perhaps it is bad, bad, bad to use plain TeX in those examples, but perfectly safe to use plain TeX in all other cases.
<br><br>The same of course holds the other way round: no amount of examples, be they two, ten or 10000, showing ways in which plain TeX does work in LaTeX will prove the general point that it does. Maybe it works fine in those examples, but wreaks havoc in any other case.
<br><br>By now we have seen that there are many cases in which a) using plain TeX in LaTeX creates a horrible problem, and b) using plain TeX in LaTeX is the best thing since sliced bread. Isn't the most sensible conclusion that sometimes it works, sometimes it doesn't. If you know what you're doing, you're not likely to need advice and if you don't know what you're doing then either don't try it all (probably best for most inexperienced users) or by all means do try it but take the responsibility when it comes to bite you in the behind (probably best for inexperienced users that want to learn something about the internals of teh system and don't mind things going wrong from time to time)?
<br><br>Just my two pence,<br><br>Yuri.<br><br></div></div>