You hit the nail on this one. I'm also a road/mtn biker and bought the Mavic to allow me a bit of self-filming during rides. I'm sure not flying it near trees and some other places that could not only crash the thing, but that would not necessarily make for "amazing" footage from the air (and that's why I bought an OSMO Mobile for, it's cool and allows incredible takes from the ground to mix with aerial footage). Anyway, I digress...
I'm well aware - and never expected otherwise - the active tracking capabilities to be 100% or even 50%. But it works OK and above all, the thing is portable. I can now take it with me anywhere to capture nice footage of the cool places I ride to show friends and watch occasionally to discover new perspectives of familiar places. Or to film friends during rides. All that was out of reach for me until the Mavic came out, I could never do any of that with my Phantoms because... well, everyone knows why LOL.
On the practical side, I've found a nice and cheap way to carry the controller + phone on a safe and easy-to-use location on the bike: a large stem-bag (these ones are from SERFAS, around $35 -
Serfas: Stem Bags). The lid is magnetic and it's big enough to carry the whole thing assembled. It's soft-lined and I use a rubber band to wrap it up if things get bumpy. Also use a stick locker to be safe, fast and easy to manipulate the Tx if necessary even while riding (as long as it's not too technical of course).
PS.: Please don't take it as I'm knocking the OP because I'm not, in any way! Just sharing my opinion and personal experiences.