Infantry is the most lethal role if played properly. AT Infantry can obliterate armor; standard Infantry are effective against light armor, choppers and infantry; Snipers can mow down enemy infantry units if concealed. They can also take advantage of calling down transport choppers and APCs and rush points or get to areas that are chokepoints for armor and ambush them.
Infantry requires a lot of micromanagement due to the fact that they are weak and vulnerable to many TA strikes such as light/heavy artillery, napalm and chemical weapons. They are also victimized by support players using Medium/Heavy Artillery -- you can see an example of this in the
Bridge Assault replay I posted.
The main thing to remember is to always stay mobile and hidden in forests and buildings. Groups of infantry shooting out of forests, buildings or moving out in the open is only asking to get creamed by TA strikes. Once you get the feel on how "long" the napalm strike or how large in diameter artillery and chemical strikes are, you can spread your infantry out to where you will only lose a group, instead of all of them at once.
The TA flow in SP was ridiculous and it is very different in MP. You are awarded TA for capturing points (5), attacking enemy units, fortifying friendly CPs, repairing friendly units and receiving it from other generous players. Ultimately, it is dependent on how smart (or lack there of) your enemy is. For example: there is a strategy called "TA Farming" that involves fortifying one of the CPs but allowing the others undefended; this will allow an enemy to capture the undefended one while they would have to blow up the bunkers on the other. The tactic behind this is that you kill the enemy on the undefended point and retake the CP, awarding you 5 TA. Rinse and repeat. Also, large amounts of vulnerable units such as light tanks and medium AA vehicles are prey to heavy choppers -- keep in mind that the more that the enemy kills, the more TA they get. 6 Light Tank kills will obviously give me more TA that if I killed 3 Heavy Tanks. If you prefer the support role and like repairing, you can get craploads of TA if you can keep your repair vehicles alive while shooting down aircraft -- good support players can pool vast amounts of TA.
Generally, getting 80 TA for a nuke is somewhat of a challenge individually in MP, but experienced players can take advantage of the enemy and farm TA from them, due to enemy army composition, the "CP TA Farming" tactic, etc; but by no means will you have an unlimited supply like you do in SP.
Also, Laser Guided Bombs are only good against blowing up bridges to delay an advance. There are other TA abilities that are more effective and anyone as Support playing Artillery or Air using Heavy Choppers can blow them up without wasting your TA.