When I do this it seems I lose about 10% total hash rate. Thats the only reason I don't.
Yeah, you'll lose a bit because the secondary miner is set with a lower aggression rate, but with the -f 1 / -f 15 I usually don't lose more than a few MH/sec. My 6950 drops from ~369 to ~363. On another machine I have 5830's on I drop from ~ 299 MH/sec to 295 MH/sec when 'failed over' to the secondary.
I'd much rather lose a few percentage points on my secondary miner when it ramps up than lose 100% of the mining when my primary is down.
You could set both primary and secondary miners to say -f 1 and it'd split the mining equally between the both of them and still have full power to one of them if the other goes down. Of course, then you're splitting your mining, so, it's a judgement call.
Experiment with two miners and different aggression rates, you can start/stop the miner to simulate one of them going down and compare what you'd get at different rates.