Consider first the manable 88 positions you have occupied in the game.
In most cases they are worthless for antitank duty unless the tank is inside of 1k. Most players cannot aim at longer distances with them. Very few players have the skill to make the anit-aircraft mode an effective feild defence. Once spotted by everything from a jeep to a Hurri MkI they can be destroyed quickly.
Now think about GV engagements you have been in along with GV attacks you have attempted to stop from a vehical feild using 17lb and 88. Once targeted by the attacking Jeep thru Tanks thats the end of the gun's effectiveness.
To give a mobile artillery peice even a few moments lease on life, you would have to convince Hitech to do away with the Icon so players would have to search a bit longer to find the gun and it's truck or half track. That would not be fair to the rest of the GVers in the game who spawn out and take their chances.
If you want to play pure anti-tank, take out the M3 with the 75mm and sneak up real close to your targeted enemy tank. I've killed wirbels with an M3 after dropping supplies and then sneaking up behind them. Mobile artillery would have a short count down life clock from the first round tracer that lit up the virtul skys.
But, I will admit that the possibility of driving out from the feild I'm defending and taking advantage of better terrain features then the illogical placements of the 17lb and 88 on most feilds is tempting. Until you consider the day you spawn your tiger towards an enemy feild and 20-30 artillery peices have you zeroed. Or that same 20-30 softens your GV or airfeild from near their spawn in point. The vTards nor their style of game play is dead yet.