Some of them, but not all of them...
There are actually a number of uses for the regular slayers.
The Reptile-group normal slayer types:
The much maligned lizardman slayer, in the hands of a human, becomes a means for a person running only crafters to acquire spined leather (especially if there is HCI on the weapon).
Snake slayers work on Yamandons and Serado, and it's a lot easier to find a really good weapon with the "wasted" snake slayer property, than to find a reptile slayer similarly endowed. IIRC, they also work on sea serpents, for fishers.
Ophidian slayers came in handy during the invasion, and during the blackrock period, when one of the major BR spots was right next to a swarming mass of ophids, that people traveling in on foot (or running in panic from BREs) would drag into the mining area, usually by accident.
Changes I'd recommend: Combine snake & ophidian slayers, and make Juka fall under lizardman slayer.
Elemental-group slayers:
Earth elemental slayers: the most popular, especially for miners who enjoy hunting with gargoyle picks. The one glaring oversight, unless fixed recently, is that Dull Copper Elementals (which are found in some dungeons and one spot in compassion desert) aren't part of the slayer family for this minor, only the Elemental superslayer.
Snow elemental slayers: These were changed a couple years ago to include Ice Elementals. Not checked them yet, but they may also work on the Prism elemental room, for key gathering. I personally have one with Valorite-hammer-level properties, that I've used whenever friends asked me to go hunting Ladies of the Snow (which are undead). The sword kills the eles in 1-2 hits, and despite NOT being a slayer for the lady, drops her in 3 hits, if one is an armor ignore. (I also have an Undead/snow ele slayer from the old Uzerran quest, but it doesn't work as well).
Blood & Poison elemental slayers: these work great for farming these creatures, which are high loot, and like any other normal-level slayer, much easier to find with great mods than a super similarly powered.
The Fire, water, and air ele slayers are the big waste. Too limited a selection, and not much one could add to them.
Changes I'd recommend: Do away with Fire, water & air slayers, or merge them with other slayers. Have it based on their 2D form and polar opposites (one for Poison, Air, Earth & snow, one for fire, water, blood and add acid eles and efreet to it, from the superslayer-only category). Add the elementals from SE to one or the other.
Repond-types:
Orc and Ogre are the most popular sub-types, for obvious reasons (well, maybe not too obvious for orcs, but my fellow smiths will understand from 4 words - Orc Brute Shadow Iron).
Changes I'd recommend: merge troll in with Ogre, (if it isn't already) put Troglodyte in with Orc or Ogre (better for balance to go with orc), and add a human slayer that works on the human NPCs, instead of them being repond super, only.
Arachnid-types:
Spider slayer has always had its uses, though diminished somewhat by the dread spider changes (why the scripters have to go ruin it for people that would hunt them for fun)
Terethan slayer has its niches, like the ophid slayer.
Scorpion slayer would be a joke, if it wasn't for Miasma (but a really sweet scorpion slayer will sell for big gold to a Miasma farmer).
Changes: Of all the slayers, this is the one I see merger of all into one group to be the most beneficial for (well, it and Demon types - see below).
Demon-type:
Gargoyle slayers should be converted to Demon slayers with the release of Stygian Abyss, to prevent confusion.
Undead: already just one slayer. If any recommendation, Skeletal dragons should be affected by these as well as dragon/reptile, because they ARE an animate creatable by necromancers.
Fey: Needs to be a craftable.
Others:
An Insect slayer (like the beetle slayer, plus solens, ant lions, etc.) would be a welcome addition to weapons.
