1. I don't want to talk you out of it. I'm running two in my maindeck right now. In this meta we have to deal with Young Pyromancer, Monastery Mentor, Thought-Knot Seer, and who knows what else. The fact that this card is good against midrange, and small creature's means that this card is currently good in the meta. The reason the card is not generally run in the maindeck on the average is quite simple. It's clunky in the opener. A skilled opponent might not run out all of their threats into your Pyrokinesis, meaning at some point, and it will happen, you will get 2 for 1'd by your own card. Games where you need to kill just a Deathrite Shaman, or just a Stoneforge Mystic hurt a lot. On the reverse side though, the deck gains an out to a lot of situations, in addition to freeing up board space.
2. I think those days are behind us. With creature efficiency only going up with each release, the days where we can afford to wait "another turn" are falling away fast. In
Davis's article titled "Funeral For A Friend", he mentions how Tarfire is becoming a necessity, and I couldn't agree more. A way to deal with early creatures who present a ton of pressure is required. Mix that with the fact that each silver bullet you run in the main requires a cut from somewhere, and you get to where we are today. I really do want to run more Mogg War Marshals in my own lists (I tend to run 2), but cutting a singleton of my toolbox lowers percentages in match-ups where they are relevant by quite a bit, even if it's still in the board.
3. Thalia is not a trap, I was just being realistic. She is hands down the best Legacy hate bear ever printed. In every spell based combo match, I want nothing more than for her to be in play at every moment. Is there more specific hate for the combo deck you are playing against? Probably, is there one as versatile and effective as Thalia? The next closest for our deck is probably Spirit of the Labyrinth, and I'd argue that gal is a league below (She is good, just not as good). I was just trying to let people know that she isn't for your fair match-ups. Your deck is already plenty good in those. Gobs have jobs, but they can't chump a Tendril's kill, Grindstone Activation, lifelinking Bargains, or even 7 Lightning Bolts. Leave fighting spell based aggro for the professionals. If I ever switch decks, it will be to D&T, and it is specifically because of Thalia.
Edit:
To be extra clear I tend to board her in against the following: Miracles, Mud Stax (Tentatively, still learning this match), Dredge, Manaless Dredge, Infect, ANT, Reanimator, Tin Fins, Cloudpost (on the play only), Enchantress, Stoneblade, Burn, Omnitell, High Tide, Sneak & Show, Mono Red Sneak Attack, Belcher, Landstill, Painter, and Turbo Depths. I even pulled that from my database of recent games I have played.
You may have noticed that only a few of those are fair decks.