It actually does, kinda.
Unfocused you're just devastating about everything up front. Just get close to any boss with full power and unleash all you've got, you'll deplete the bosses' health in no time. Focused you get to cover a huge range (entire screen with full power) - which comes very handy in levels - but the drawback is that at bosses you'll have to suck it. Even with full power you can damage most bosses rather well if you unfocus a lot from the bottom of the screen; which isn't very easy if a lot is going on though.
Were you to invert the shots, you'd get a very overpowered shottype raping about everything that comes your way. So you gotta balance it out a bit. Now with eight bombs stocked at once, you have quite the chances to avoid dying - if you're good at the stage portions, then you have the perfect insurance. 
Unfocused MarisaA at full Power is slightly weaker than ReimuA on average, because let's face it, times when you'd be able to get the outer two streams to hit for any appreciable amount of time are extremely rare.
The last sentence of your first paragraph is kind of making my argument for me. This isn't as much of a problem on lower difficulties, but on modes like Lunatic there's
always a lot going on. Lunatic in particular has the most consistently dense danmaku in the series - in fact it's dense enough that it makes Marisa and her larger hitbox remarkably close to unusable (at least, more so than in other games, since it's obviously still possible to clear the game with her; point is that it this combination makes things significantly harder than they need to be).
MarisaA's bombs only last half the time of almost everyone else's bombs, so in terms of invincibility time you need to use 2 bombs to match those of others. Problem is, the way the Power items are revalued, it's actually
harder to regain the equivalent number of bombs stated above - she has twice the Power capacity, but small Power items are not twice as valuable as before (they're only 66% larger), and large ones don't change at all IIRC.