Mortals do lose out in combat against the supernatural, but they gain majorly in other aspects, and are excellent to have for storytelling purposes.
We've had a two-year campaign running, starting from a relatively low level and currently at Chest Deep. We have honestly found that there's advantages and disadvantages to both mortal and supernatural characters, and that while mortal characters lose out in some respects they definitely gain in other respects.
Mortal characters do, fairly majorly, lose out when it comes to facing off against anything with supernatural abilities (eg. inhuman strength/speed/toughness), or when it comes to anything magical (eg. veils and glamours). The lack of The Sight, the inability to do any kind of shield, and the lack of any comeback against magical abilities (eg. spells) is a problem when fighting.
However, this isn't necessarily something that other characters won't face (for example, werewolves have the same issues against spells), and actually can really benefit the game! Having a character who won't blindly charge into combat and actually has to think their way round a problem can be excellent. Some of the issues can also be got around; handcuffs, for example, are very good against the Fae...and we placed a True Love aspect on one of our mortal characters (which entirely fitted their backstory) and led to some very nice advantages against White Court vampires.
Mortal stunts are also a nice bonus; having a gun gets you quite a way in combat, especially combined with a good alertness bonus. Not having the supernatural abilities is not necessarily a complete show-stopper.
The place where mortals really get the bonuses is in the stunts. Marcone is a good example; a reporter like Susan is also a good template. One of our characters became pretty central to the story because of their bonuses in investigation; if you had this character skip a scene, suddenly you've got half the information you would otherwise have! Things like resources, contacts and driving can be very useful, and the ability to create your own stunts and aspects is definitely a tool to work with. This does obviously have advantages for combat bonuses, but can be brought to play for plenty of other aspects.
In my personal opinion, you need mortals in your game to give some balance to the story. If you have all supernatural, you run the risk of ending up with a bunch of thugs who fuego first and can't ask questions later. If a mortal knows they can't compete, they have to think of other solutions. That Faerie you're after for murder...can you arrest it? That troll you need to stop...can it be diverted or bribed? Can a situation be turned political, can a diplomatic solution be found, can you sneak or seduce your way to success? And this goes for the high-powered end as much as the low; the GM should be tailoring the game to the character's powers, but at the high end, you're more likely to be interacting with people like Mab, and that definitely calls for some mortal abilities.
And a personal opinion
The place where we actually really struggled with the Mere Mortal template was explaining their knowledge of the supernatural! If they're not automatically involved in the world, how do they know about it? And how do they avoid being sucked in? One of our Mere Mortals did end up taking a Mantle, but the other did stay mortal. Have a think about how your character knows what they do, or how they can gain that knowledge without becoming affiliated (or put in danger) by the supernatural community.