I don't see it considered very often, but hear me out:
-Only 25 points (5 points per TL lance is points efficient)
-Good anti-tank weapon on a durable platform
-Can also shoot at non-vehicles, AP2 compensates for lack volume a little.
-First turn: Often you want to scout + Run first turn anyway, essentially removes the (probably false) choice of wanting to shoot cannons instead,
- This unit is going to be absorbing firepower and is there for board control. The 36" range on Cannons is nice (granted), but if you are near the middle of the board, something should be in 18" range. If you aren't, you're likely sacrificing board control you shouldn't be.
-Splinter cannons are plentiful elsewhere
- If I am trying to get in combat with a vehicle/transport HL to remove it gives me two bites at the apple (in case I have a chance of failing charge), and often I just want it dead anyway.
- If it is a transport, I get a shot at 2 kills a turn. Particularly tasty against gladius.
- Makes it slightly harder to choke the unit off with sacrificed vehicles.
- 18" vs. 36" shooting effectiveness
-On a turn where charging a toughness value, if I'm trying to get in CC with a unit out of a transport, I often don't want to shoot for fear of increasing charge range. This means the points are wasted.
- If charging a vehicle towards a board position you want to be at, blowing up the vehicle prevents that range gain. (Though, there is always the risk of failing the charge anyway)
- Changes the target priority of the unit as a whole
- Less poison. 5 TL BS 3 vs. 30 TL BS3, variance and volume highly favors more wounds on the cannons.
- Costs points
Context might help here. The rest of my list is a lot of venoms, reavers, and a unit of haywire scourge, no ravagers.