Magic Is Too RandomThe most common objection I hear to the DCC RPG (well, it's between this and the funky dice) is that the spellcasting check necessary to cast any spell in DCC makes spellcasting far too random. I think there's another piece of this puzzle that the "too random-ers" leave out: Spellburn. I've noticed that players new to the DCC wizard and elf often don't Spellburn and just trust to the randomness of the dice roll. This trust, I feel is why they get let down and is the genesis of the opinion that casting is too random. The unwritten rule of spellcasting in DCC is that if you absolutely, positively want that spell to produce the effect you want it to have, you've gotta Sepllburn to get it. Even if you manage to roll an awesome success, Spellburning can almost never make a situation worse (unless you roll a "1"), opening up those upper tiers of the casting chart for the particular spell in question. Spellburn turns your physical Ability Scores into a power reservoir that becomes the both the fuel for and the limitation to the greatest heights of magery. Want to be a badass wizard? You've got to pay for it.
And then of course there are the spell tables. The better you roll, the better your spell result. While this seems to put wizards on par with warriors and other combat-y types, rolling for their success, I get that most folks who are used to playing wizards are used to the "point and shoot" nature of spells in old school editions. I shouldn't have to roll for a spell. I'm just using up a spell slot. Some spells scale up with the caster (Magic missile comes to mind with its ever-increasing number of missiles) but some do not (Sleep for instance). There's nothing to differentiate a 20th-level Magic User's Sleep from a 1st-level nobody's. DCC takes a very different view, making the 10th-level wizard's spell potentially much stronger than the 1st-level wizard's because of his Spell check modifier and the application of the spell tables. However, should he channel enough power through Spellburn, that lowly apprentice might be able to rival the master! And you're not burning through spell slots, but instead your own vitality! In my opinion, that shit had better count and better have a damn good chance of an awesome result. Thus, spell tables. You get better at magic as you go up in level (having to burn less for lower-level spells at higher levels) to introduce greater levels of choice in how you tackle spell casting at lower levels.
Even if one concedes the addition of spell tables, why do we need Mercurial Magic? In short, we don't. In full, Mercurial Magic isn't expressly needed for magic to function in DCC and could effectively be ignored, BUT doing so ignores a huge source of flavor that you roll once per spell (preferably at the game table, not beforehand; that "but I should know my Mercurial effect before I cast" thing is way too boring) and informs the choices that you make as a player when you start thinking about casting a spell. For example, +Donn Stroud rolled a "97" for his Flaming Hands spell (or, as he calls it "Phoenix Phingers"). That result reads as:
Necrotic drain. The spell is powered by the energies of the living. The nearest creature (other than the caster) takes 1d6 hp of damage per spell level. For every 2 hp lost, the spell check result is increased by +1.You had better believe that Donn thinks and plans before he chooses to cast that spell, especially after he killed a party member by standing too close while trying to start a fire (no joke).
Much of what I've written here is about choice. You choose to Spellburn to get a result. Or to get a better result. Or to burn Luck to bump that spell up just a little bit more. You have a more interesting choice when you have to bear in mind a spell's Mercurial Magic effect than without. The "too random-ers" seem to ignore the fact that, rather than just relying on randomness, magic in DCC is about making important choices and risky decisions; the more you're willing to gamble, the better the pay off is. If anything, these choices are about mitigating randomness and the existance of the randomness in the rules is to make choices more meaningful.