Fixed gears not being able to coast has always been made a bigger deal than it really is.

When you are on a fixed gear, the only difference between it and a freewheel in terms of coasting is that your feet still move. However, you don't have to be applying force to the pedals. It helps to at least move your legs with the pedals while coasting' on a fixed gear, in order to not sap away some momentum by forcing the bike to push your legs, but it doesn't feel like you're constantly pushing the pedals either.

Of course, it depends on gearing going a decent speed down a hill is different in a shorter gear. In a tall gear your legs must move quick, but not that much out of the realm of your normal spinning technique, but with a gear like 34/18, if you're going down a hill on the road, then you'll be spinning much faster than you'd probably like. Giving the feel that the bike is 'pulling you' down the hill.

Personally, if you want something you can ride on the road and on the trails, I would think it would be better to have a different gearing for each, and carry them with you if you do both on the same ride.

For instance, for road riding using a 46/16 set up, and for mountain riding, use a 44/20. Bring the 20t cog with you, and a chain to accomodate.

Or if you aren't too picky about your chain, just do two master links and remove/add the necessary links as required.

Since you're not changing the chainring in the above example, you only need an axelnut wrench, a lockring tool and a chain whip. Which is something a lot of people already carry. And if you have a fixed/fixed hub, you don't even need the lockring/chainwhip, just the extra bit of chain and 15mm wrench for the axel nuts.