"If you want something done right, do it yourself."

If you tie something off, you no longer have a sense of it--not to mention that it can be unwoven instead of having to be ripped through by brute force. But imagine a tied off dome of air. If you do that, you will have no warning when it is ripped through, until the lightning bolt suddenly hits you on the head. Even if you do notice the weave coming apart, you would not have enough time to reform it before you are struck down, so some things are simply too vital to be left tied off.

Furthermore, I think you are underestimating just how impressive a feat it was for the Asha'man to form a dome of air. After all, for Aes Sedai it is a simple process. They form a circle, and then weave a giant dome to defend themselves. Asha'man cannot link together like that, so each had to create a separate segment of the dome. That left no room for mistakes, as even one segment collapsing could leave them open to attack, so they had to be extra careful and maintain those weaves at all times.

As for your other points, there are no doubt better things Asha'man could do than simple lightning bolts or fireballs, but some of the weaves Rand showed them might be too difficult for most of them to try repeating on a hectic and confused battlefield. As for the ones at the Black Tower, they might simply not be teaching anything more advanced than basics weaves now, as Taim prepares to massacre those siding with the Light.

Also, I don't think it's possible for a column of fire or be tied off. It has to either be burning on its own by then, from some sort of fuel or wood, or it would go out, because fire isn't a simple sphere of light to be tied off. Fire would need constant input of energy and Power, because otherwise it would go out.