Interesting Idea. Artillery sure is a great way to deal with Clanners (and everyone else...) but making it the mainstay of the armor brigade of a RCT is, imo, a bad idea for two major reasons.
1) Artillery needs a lot of protection. Since artillery can be such a devastating weapon, it becomes a priority target, especially against the Clans, who see it as honorable and would probably react very fast and brutal to the large scale use of artillery. Therefore, a screening force is needed, that is able to deal with fast moving enemy forces, be it fast Mechs, VTOLs, Hovers or ASFs. When using FedCom RCT structures and the typical BT Universe mindset, the only reasonable way to form such a screening force is using (heavy) tanks.
If one would use Mechs for this, Mechs would become a supporting force to the conventional forces, since pretty much the whole Regiment would be needed to guard the much larger armor brigade.
2) Artillery is really messy. It's the least precise weapon in the inventory of any BT army and causes a lot of collateral damage. Unless the enemy is willing to only fight in open fields without anything valuable nearby, artillery can become pretty useless, if you want to take something like cities, factories or anything else in a semi-intact state.
Plus, if your own forces are engaged in "close" combat with the enemy, your armor becomes useless as well.
To conclude, artillery can be a very potent force, but it's one of those weapons, which are actually able to achieve overkill. Bring to much of it to a fight and it turns from a force multiplier into a liability.
Now, to answer what tanks should be able to do:
In my opinion, tanks should be able to perform all "lesser" tasks on the battlefield, to free up the Mechforces for the high profile (mobile) fighting. Stuff like: Providing artillery support, guarding the artillery, supporting Infantry forces in defending the ground taken by mechs, finishing off crippled enemies, etc.
Anyway, that's my 2cents worth...
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