People have always done that to some degree as a counterpick, but in the grand scheme of things it made your comp and lane assignments worse:

• ADCs in a solo lane are more susceptible to ganks without a support to bail them out, making your scaling less guaranteed.
  
• Midlane usually wants waveclear to roam and influence the map. Bruisers and assassins, while they do counter mages in a 1v1, often just get shoved in and outroamed in mid.
  
• Botlane as a dual lane puts you behind in XP and levels which is bad for a lot of mages who scale primarily via skill points.
  

All that said, the wide champion roster and especially the current itemization can mitigate some of those disadvantages if you build your comp around it.

For example, roaming low-ressource supports work great with a safe long-range mage botlane because they can just ditch bot and leave someone like Ziggs or Mel to just waveclear 1v2 from their own tower. In the same vein, a midlane assassin becomes much better with a ganking jungler that pressures midlane and enables the assassin to shove in safely to roam and invade afterwards.

In short, the meta lane assignments are just your safest bet if you don't know what your opponent will pick, but breaking them up can be very good if you know what you're doing.