Well...
5/6 applies to "Regular" Clan vehicle troops. Vets are still 4/5, elites (rare outside CHH) are 3/4.
Also note you can elect to have random skills generated, which could add a bit of flavor. Talking about the tables on p.273 of TW. Seems to me a client may eliminate the cumbersom task of rolling up a large force, thus improving fun.
That said, the 5 gunnery isn't that crippling, but does adjust tactics.
First off, you have access to quite a few pulse lasers, and generally speaking, more firepower per ton. Secondly, vehicles are generally considered outside zellbrigen, so they can group fire in almost every circumstance.
The 3rd aspect you already hint at: super cheap firepower. It forces close proximity combat, but especially tracked vehicles can often engage targets in short range, and without movement that turn. Presuming the target made a 2 move-mod, that's a 7 to-hit.
Now, it still sucks, and especially hovers really feel the bite of the 6 piloting, but to discount them outright seems to go a tad far.
Still, you could elect to ignore canon on the skill point.
Paul