I think the idea behind suggesting assist was not to include it to that macro of yours. The idea is that, since you can't put <t> and <tt> to same macro anyway, you only have the natural Regen somewhere in your crossbar. Then you have somewhere in your controller the assist command. Now, if you are in a situation you want to cast -anything- to the main tank while you target the boss, you first hit the assist, then cast whatever, Regen or something, the click assist again to get target back to boss.

It's solution to the targeting issue with controller, not to the macro building in itself. It also gives you easy way to target the main tank for all purposes, not just one specific action.