The guys got a point, if he is holding a lane, then lay mines on those creeps. Thats one of the points of the skill. AOE damage for fast levelling. The wave is cleared and you can push with your wave onto a tower.
If a DG comes back off and wait for mines to refresh, you have already taken out the wave so you have full xp and gold.
Running in to lay mines on 2-3DGs as Reg is pretty dam stupid, unless you have backup, even a hint of focus fire will get you creamed.
Also, you have to be pretty dam stupid to run straight at a Reg camping a flag, as you should know he will have mined the flag to shit. So people who get "ganked" by a flag camping Regulus really need to learn a little bit more.
Being told off for doing it is pretty annoying, altho the relation to Dota I don't know 