Specifically in team games, flares synergize well with abilities that require continuous line of sight like zeroing artillery, something recon passes do not. The fact that flares have no obvious audio or minimap indicators makes them far harder to react to then single pass recons, and there's also the fact that the recon runs take time to arrive, while flares are near instantaneous. This can mean the difference between getting your troops spread out in time or eating a light artillery barrage.
The problem is not reckon planes them selves but being stock.
I have already said that the CD of flares need a massive increase but using flares and off map not only requires a specific but also a second one for the off.
If in your opinion the problem is there is little warning from flares than simply add a warning.
The stock reckon planes multiply the effectiveness of most UKF/USF off map. One wants to use P47/Typhoon loiter ? add the reckon planes and you get both vision and more to divert AA fire.
My point is that reckon planes should not be available stock.