I'm in the camp of people who believe expacks should be allowed as primary weapons. I agree, however, that it would be ideal if FF expack damage was full ( self damage at least). It's very annoying when the person standing next to you detonates the expack and survives while you die. But I digress.
The expack rule puts engineers at a severe disadvantage, particularly in close combat. I've heard a certain Admin suggest that the Engineer should empty his clip before even considering using Expacks against Non-Vehicles. This means almost certain death. Even if you are lucky enough to hit your opponent (which is tricky) you often have to shoot them up to 2-3 times before they will die ( a virtual impossibility with slow reload). Obviously you would expect the engineer to lose a strong majority of close ranged battles, and that is still the case with expacks allowed as primary. However, the expack gives an approaching Infantry an incentive to keep his distance. It's that distance that allows the Engineer to actually succeed in getting that head shot with the rifle.
If the Medic knows that the engineer is handcuffed from using his expack, he can simply dance his way to close range while spam healing and then gun him down with ease. Currently since compliance for this rule is so low, this is not as likely to happen.
All the other classes, including snipers have the ability to use nades offensively and defensively which is highly useful and represent a decent percentage of the total kills for any class. Its not uncommon to see skilled players using Anti-Tank class, racking up a large percentage of their kills with nades and near the top of leaderboard. As I have implied, the hapless engineer has little chance of dissuading anyone from approaching with his pistol and rifle.
Additionally, even with the current situation of engineers not complying with expack rule, you do not often see engineers atop the leader boards (with the exception of Pilots, Tanks, and Pacific maps like Iwo Jima with the superior rifle) which suggests that expacks and engineers are a useful but not overpowered class. (Their utility certainly can't be denied with mines and wrench )
I suspect the argument against expacks is more of an emotional one than one based on any unfair advantage. Many people dont' like getting expacked; especially repeatedly. It makes them angry. But if you take a step back and look at things, they actually help provide a more balanced gameplay then not.
In summary, removal of expacks as primary weapons is a mistake and only serves to significantly reduce the incentive for non-vehicle-driving players to choose that class.
The other problem that I will just touch on briefly: The rule is so poorly enforced that it effectively only penalizes the small minority of regulars that agree to comply, thus creating a frustrating experience for those people when they are expacked themselves
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note: I have Lyme Disease (with a mind like scrambled eggs at times) and I have to go back and edit anything I write so if something doesn't make sense please let me know.