In short, there are just better instants in both red and blue you could be playing for three mana, especially in spellslinger strategies where mana is precious until you hit a critical mass of rituals
And even if you don't go crazy, sac a bunch of tokens, get a bunch of Blood Artist triggers, and what have you, you can still easily end up with a pretty big flyer for four mana or give/proliferate it counters through other means
Even though $180 for a raw card is expensive, this card is super unique and rare enough to want to add to your collection and hold onto for a long time
Really any Dimir deck that uses cards that reduce power and toughness could work well with this card and others like it
Any ramp deck, later game deck, and control build (particularly one in W/R) will benefit from the Tower