^ Actually, in Lucario's case, you really do. Even with Iron Defence, an untimely Critical Hit will be the complete death of you. OHKO abusers will walk all over anything without Sub, which has actually been the end of many good Tower streaks, including my own. Substitute is basically the ultimate answer to Hax and something taking as long to set up as Luke does is more or less Haxbait. Flash Cannon is a fine move for this Luke; 120 Base Power and only 4x resists held by Heatran, Empoleon and Magnezone. At +6 it will rip through almost anything.
And if it counts for anything, I found myself being grateful for having Sub to ward off the AI's hax, status, etc. many times during my run with this same Luke, before being stopped dead at 144 by a Gliscor which got in some Guillotines while my Sub was down.
Anyway, onto the team. Tbh Pokémon with Hyper Cutter do not pose a large enough threat imo to warrant the use of Reflect on Latias. Pinsir locked into a Bug move gets lol'd at by Lucario, while if it's in a Fighting move for some bizarre reason, Draggy can take it well enough. Any of the Hyper Cutter Pokémon except for Mawile are only really a threat if they carry OHKO moves anyway, which don't care about the Atk stat. Looking towards Clear Body Pokémon, Metagross 4 is a threat, yes, but for this I'd recommend you go with Flash anyway.
892 | Metagross | Adamant | Quick Claw | Meteor Mash | Zen Headbutt | Earthquake | Explosion | HP/Atk
If it blows up in the first turn then yes, you're in an awkward situation but there's nothing really you could have done. The most likely thing for it to do against Latias is to throw Meteor Mash at you. This deals out 41.7% - 49.7%, which does allow you to get in a Flash to play on its pretty mediocre accuracy. This is only a shaky fix, I understand, which is why I would recommend upping your Spe EVs to 44 to outspeed said Metagross even after you've tricked it the Scarf.
Basically what I'm trying to say is Flash > Reflect, because Metagross is the one Pokémon which is of great threat to you who's Atk cannot be reduced, and imo one Pokémon does now warrant the use of Reflect when 44 Spe + Flash does the job fine, not just for Meta, but for anything else you can come across which can be Charmed.
I disagree with Outrage on Draggy; when I used Latias and Lucario together with Swampert, I found Lucario doing most of the work, but occasionally Swampy has to come in. Replace that with Draggy and you run the risk of getting confused after only two opponents have been KO'd, and then causing yourself +6 Atk confusion damage while the AI proceeds to break your Sub and then finish you off completely, especially as you may well hit yourself again. I'd really recommend Dragon Claw.
Hope it helps.