LDF vs LMR
The primary reason for recommending LDF over LMR is noise.
In a duplex situation (a repeater), noise will be heard on weak rx signals if/when the LMR cable is moved/flexed. The two metals in the shield of the cabble rubbing together will cause rx noise.
So, if the LMR is tied down so it doesn't move, the LMR is just as good as LDF for your application. However, if the LMR is subject to any physical movement (wind?), you'll hear crackling "popcorn" noise on weak signals.
-- Ray --
PSR500, PSR800, IC-R7100, IC-R20 and a pile of Amateur Radios