No, it would totally **** to have a bike handle tiller. I've had a Stihl bicycle handle trimmer (gave it to a good friend who needed it more than me), and own a few Kombi machines. The bicycle handle is fantastic for string trimming, and probably brush cutting, but doesn't lend itself to much else. It lets you hover the trimmer head at a height and counter the cutting torque with the handles, but you have very little vertical control authority. Every single time the tiller tines contact the ground, they'll kick right back up in the air, and you'd be relying entirely on your wrist strength to keep the tiller cutting into earth. I had (and sold) the tiller attachment for my Kombi, and found it took too much effort to keep the tines cutting even when I could lean on the D handle.
I also owned a couple of Stihl Yard Boss machines (Stihl's take on the Mantis). They place the motor by the ground with the handle bars up top. It's workable like that, but still gives you a workout, which is why they sell a weight kit.