If you choose not to (or can't) cast the card, it remains exiled with no time counters on it, and it's no longer suspended
Sisay, Weatherlight Captain is an established deck in that format, and this card seems like a slam-dunk there
The most known trick with Brainstorm is pairing it with a shuffle effect like a fetch lands ability to shuffle away the cards from your hand you put back on top
Destroy target artifact, enchantment, or creature with flying
Urborg can make your swampwalk creatures unblockable, and it can make your colorless utility lands tap for mana
These battle cards will be similar, and while evaluating what the front half does is going to be the much more important part of the pair, even the threat of the leftover value of the potential backside is going to change how games play out