And standing in front of a dummy that does not fight back doesn't help one become a good dps in real trial either, but it does provide some numbers to know if your gear and sustain management is good enough.

Recently a tank in my guild ignored all the advices and upgrade max a full set of reinforcement armor master, claim that with max resistance he don't need to block that much so no need for sturdy. He wanted to bring that tank to tank axes in vAA and vDSA too, but no one want to take him. He later realised his mistake after wiped us a few dozen times in Bloodroot no HM because his resource management is horrible. If he cannot hold block to 2 stone atronachs in Bloodroot (the 3rd one die immediately after it spawn), how can he tank axes in vAA?

Before training dummy, dps had to ask for a tank to help them test parse on that gaygoyle in Spindleclutch. Now we can do it ourselves and it's much more easier. Same with tank. Currently one of our guild require tank to be able to tank axes in vAA or kite the last round in vDSA, but it take 11 or 3 other people going thru a whole trial just so a tank can test his sustain. It's just annoying.

It would be useless. Healing in this game is not about spamming heals and tanking is not about holding block. You can stack enough heals with any gear that makes sense, but it doesnt make you a good healer. Good healer is someone who doesnt just heal, its all about buffing, helping your group with resource sustain etc. A dwemer blade wont help you to achieve 100% combat prayer and ele drain uptime... Unlike dps dummy which actually teaches you to get more DoT uptime and improve your rotation. That's a fundamental difference. Even a magsorc dd with a pet can spam heals, and nightblades do so while dpsing, so pure hps is worth nothing as long as your dds and tank are resource-starved and not getting buffs and debuffs. This "healing dummy" will just teach new players to spam heals, which is useless, anyone can spam that healing springs button without any sort of training.
Tanking is also not just about blocking and eating damage. A lot of content does not require permablocking, and there's a number of things you cant block (oneshot mechanics in dungeons). There's also certain attacks that have to be interrupted or dodged.
And its really not smart to just eat damage. Sometimes its better to just dodge or sidestep, especially since the new dlc dungeons and trials have a lot of oneshots/almost oneshots. Also, building for infinite hp and permablock will exclude you from vet content, especially trials where you're supposed to wear sets that are buffing your team.
Another imprortant thing about tanking is positioning (stacking adds, placing the boss so it wont cleave the group etc) and dwemer blade will not teach you that. Again, dps dummy is different, it teaches you to use more than one button at least.

However, building houses with traps, mazes and jumping puzzles could be a fun idea. It's just useless as tanking and healing practice. And not only that, it would teach newbies bad habits (overhealing/permablocking and taking unnessesary damage when you can do something more useful) and would certainly exclude them from more competitive groups (it wont be very convenient to re-learn when you decide to start vet trials).