As far as combat spells being useful as preparations, I'd say they would be, regardless. However, remember that when you do a 'contact' trigger, the next living being to touch the lynchpin sets it off. Including you, if you pull that knife out of your pocket (or even pick it up to place it in a scabbard). Contact and timed triggers are best used as traps. With some forms of preparations (the onmyudo charm, for instance) using tongs to put it in a bag would keep it safe, so you just have to reach in the bag to activate the spell. But knives and arrows? You touch it and you've already set off the spell. As for a 'Molotov fireball', the lynchpin would be damaged by throwing it, ruining the preparation.
Now, if you're asking how to use a combat spell as a preparation, that is simple enough. WIth a command trigger, you can still do a LOS spell easily enough, since you designate the target when you activate the charm. With area effects, they are best used as traps. Any of the trigger conditions work well for this, essentially turning them into mines. In fact, I'd say that area spells are even better as preparations, especially to cover your trail. Slap a charm on the wall as you run through a door, corpsec on your heels, and wait for them to get through... Fireball. And a timed Powerball/Fireball makes for a WONDERFUL distraction when sneaking into a compound. Who will notice someone cutting the fence when six explosions go off simultaneously on the other side of the compound?
Of course, I personally think that spells like Barrier and Mana Barrier are even better as preparations than as spells. And I can see great use for a Petrify preparation, as well, especially when you let the team's stealth-meister slap it on the guard's back, so you can give the command, or other such things.
The thing about preparations is they have to be prepared ahead of time. This is different from spells, where you can mix and match and keep casting until you fall down from the drain. Different setup, different tactics required. Specifically, you need a different mode of thinking to make the most of preparations. Most combat spells, however, I would agree would be better suited as spells rather than preparations. However, since at higher priority levels a mage (or mystic adept) gets two magic skills at up to rating 5, if you have Spellcasting 5 and Alchemy 5, with specializations for your favorite school of spells, then you're doing quite nicely, don't you think?