I think a chunk of the dislike for the PTD skills (especially Slay) as well as Assassinate and Terminate is how much less beneficial they are when compared to other limited use ways of obtaining similar results, i.e. spells and alchemy, and particularly magic items.
Although High Magic has mad this less true, the majority of scholars are going to be buying spells a majority of the time, because that's really their best option, and it's not that close. Yes, formal magic is now more attractive that it has been in the past, but in terms of raw output it's much more beneficial to gain an extra 1-9 than the build (almost) equivalent 6 formal ranks. For the martial classes, that's less true. With the the option for passive damage increases always on the table, the benefit of exceptional offensive options in limited availability is questionable over the option for higher basic damage and an improved rate of defensive skill gain. For scholars, defensive vs. offensive is a daily decision made every logistics. For a rogue or fighter, it requires a catalyst ritual to change. What's even worse is that choosing to grab PTDs (or the Rogue equivalents) really requires an "all in" decision: having just a few is definitely worse than having a lot or having none.
This is compounded in the case of the cross classes. For a scout, adept or templar build choices are even more important because everything costs more, and you have less of everything. If you're 25th level and gaining ~1 build per weekend event, do you prefer to put your next 10 build into 100 points of damage across 2 slays, or an extra disarm, magic armor, bind, shun and spell shield, with the opportunity to tailor those spells to your needs every day? If it's going to take you 41 build to get your next dodge, how much damage does an assassinate need to do to make it worthwhile? Slay and assassinate are especially bad here, since they only function at their highest potential when bought in bulk, something that is deeply difficult to do when your entire character concept involves spending 40-60% of your build on things that have nothing to do with your slay and assassinate damage. If the two skills stacked with each other, they might be cost effective to scouts, but even then they would be among the least effective options available to you.
That's because they're weapon blows and are therefore subject to blocking, parrying, damage type immunity, and not being magical. Let me explain: I play a campaign villain, a vampire, for NH. He's a fighter and has over 500 body points. He has very large numbers of defensive options, which can be sorted into three categories: physical defensive, magical defensive, anything defensive. Since his body point pool is very high and plentiful necromantic healing is available to him, damaging effects are of secondary concern to effects which remove skills. In order to be taken down, all phases and dodges must be used up (anything defensives), all self-restorative such as drain options must be eliminated (anything defensives), all cloaks versus one effect group able to incapacitate him must be used up (also anything defensives), and a delivery method for landing the decisive effect must be cleared, i.e. all of physical or magical defensives must be expended. If the decisive blow comes from a physical attack, magical attacks that only used up a magical defense were effectively wasted, and the reverse is also true. Because martial skill attacks are limited to disarm (minimally effective), shatter (ineffective), stun limb (modestly effective in bulk), damage skills (as mentioned, typically ineffective) and straight to dead skills (highly effective), only eviscerate and terminate are worthwhile uses of those physical defensives, and a large number of attacks is needed to burn through them all. Magical delivery offers more options that need to be defended against, both in variety of effects and in chances to deliver them, because a scholar or even cross-class caster will have far more options for spells to through than a similarly leveled fighter or rogue has skills and because you can make times per day magic items for spells.
I cannot stress this point enough: one of the biggest reasons that weapon attack skills are not highly valued is because anything they can do ritual magic allows anyone to do build free. If you know either you or your buddy is going to be able to throw death/prison/purify/confine/sleep/whatever enough to take care of your offensive needs, it is wasteful to spend your build on offense. You skip the slay, because that means your next parry is coming that much sooner. You get spells instead of assassinate, because healing is more useful than damage. You pass over eviscerate for herbal lore, because you can stockpile laugh gasses for when you need them, but if eviscerate gets magic armored, it's gone for the day.
