The life gain puts you out of burn range, the two bodies block, and the fact that you do get two bodies helps to guard against removal
There are only seven ramp cards total (two One Jump Ahead, two Gramma Tala, and three Mickey Mouse, Detective) which can make it difficult to ensure you get them in your opening hand
Id actually consider running it if it were a 2-mana 1/3 with the same ability, but as is, its just a fun card to consider in samurai and attack trigger decks
If Tawnoss Coffin becomes untapped or would leave the battlefield, the exiled creature returns with everything attached to it
Something like having a 5 mana scry for 2 on a land would rarely be activated and thus rarely worth it over the basic land unless it had no drawback at which point it would see play everywhere with no reason not to
It can eventually become unblockable of course, but paying 4 to make your 1/1 unblockable is going to feel really bad sometimes, especially because the Thief is so darn easy to kill