I think for the most part if you use both methods at the same time then you won't fish either very effectively. If you're concentrating on the float then you're not concentrating on the lure, and vice versa.
When lure fishing there's not many times that you need to spend 10 minutes in one swim. I usually don't spend anywhere near that amount of time unless I can cover a lot of water from one spot, or know the fish are gathered there, or the number of swims is limited. Staying in one spot will reduce the fish you cover with a lure and so reduce the fish you catch.
That being said, there are times when I occassionally do it. If I'm travelling a long way to lure fish and there's a chance the river is coloured then I'll take a bait rod in the car. It's saved me a few wasted trips and has caught me fish when the river is out of sorts and lures aren't working. When that happens, I'm bait fishing and chucking a lure in hope rather than using lures effectively but in that situation 2 bait rods would catch more fish.
It's all about optimising fishing time and using the best method for the conditions.
On lakes when you're fishing static then having a bait rod on a buzzer and chucking a lure around isn't a problem, as long as you don't wander off away from the bait rod, which is very tempting after chucking a lure around for a short time.
On boats I find having a float out is a real pain when lure fishing. You're constantly looking over your shoulder at the float and wasting a lot of time not concentrating on the lure. With lures you need to know exactly where your lure is and what it's doing to consistently catch and the float becomes a distraction rather than a help.
