TM Usage

a) No. Infact, in the current meta the only defender that is potentially worth powering up is Blissey and even that is dubious since even a big fat Blissey won't deter anyone but noobs in this meta.

b) I have long argued that TM's an especially fast TM's will accumulate over time to the point where they become near worthless. At present they still have great value to most players who don't raid hardcore all day long, but over time you will struggle to even find pokemon to use your TM's on. In that case, you could use a TM on a defender. As of present time, I probably wouldn't.

a) That's for you to decide. I can say for my part that I have a handful of mons powered up to level 39.5. Most people swear that powering a mon up past level 30 is a waste. It's not. However, if you think it is, then don't bother. It doesn't have to be any more complicated than that.

I know my maxed out Dragonite, Golems, Tyranitars are responsible for quite a few extra damage balls in the raids. They also make tearing down opposing gyms faster. It matters because once you gold out a gym it spins like a 7th day spin. Do a few of those in your local area and it's better than the "good ol days" before gyms spit out 3 items.

Lastly, if you play this game then part of the fun of the game is fun. It's fun to have a 3500 Dragnoite, a 3100 Blissey, and a 3100 Snorlax, and now a 3300 Zapdos.

It costs time (a.k.a. stardust) to do this, so if you're not up for the commitment then obviously stopping at level 30 is good. If you're into groupthink and feel that there is no reason to level a mon over 30, then don't level any over 30. Doesn't matter.

b} I used a tm on my Blissey. Arguably the worst use of a TM since Blissey's moveset is nearly irrelevant. Yet, I had a lot of TMs and since Blissey is a commited defender I gave it the best defensive moveset. Other than that I changed about a dozen sub-par moveset mons to A-rated. Including my first two dragonites in the game (SW/HB, SW/HB). Other than that I don't feel there is currently a use for TMs on defense since defensive mons are almost irrelevant.

That being said, remember that people loooove to think they know the game inside and out and will tell you things as though they know anything more than you do. The current "meta" is just that, current. A few months ago everything would have been different. Defenders were much more important. Raids weren't around and therefore certain mons were just meh.

The meta changes. Slowly, but dramatically. Today's best pokemon could be tomorrows so-so. The game itself changes. And with those changes the demands on specific pokemon change with it.

So the short answer to your questions? It's a game. Do what you want. I have friends who max out useless mons (Slowking, Kabutops, Crobat) because it's their favorite mon. They want "style points" and aren't so obsessed with min/max the best mons. And I respect that. How many level 37.5 slowkings do you come across in a gym?

Have fun! Do what ya want!

1) Only power up your IV100% Pokemons or Blisseys (so that low level second accounts timed out against the Blisseys).

2) People are already TMing for fun especially with Fast TMs.