There is just so much in this thread...
Why play a class other than infiltrator?
In regards to the question by OP, he pretty much answered his own question. You would want to play the other classes because they don't play like infiltrator. And 3 of them are not as unbalanced. I agree with others that you can make a good case for Vanguard being the actual best class in SP, given shieldgate, Nova, and IEB ammo. Sentinel has become unkillable with imported MP changes to TA and Fortification via the patch. The pure classes lag behind these in power by a small amount, but they are still more than powerful enough to get through ME3.
ME3 vs ME2 Insanity Difficulty
Whatever the devs may have claimed, the game is easier than ME2, and it is also easier than the starting portions of ME1 on Insanity. They did in fact improve the AI somewhat for ME3. The enemies are somewhat more intelligent about their positioning, they have the advantage of smoke grenades, real grenades, and ok damage. But there is a lot more to total difficulty than just the AI.
One of the drastic differences between the AI in ME3 and back in ME2 is the accuracy. In ME2 the AI was practically always locked on and if you were in line of sight, you were going to be hit. This is not the case in ME3 where enemy accuracy starts off poor and then moves to accurate while you are in the LOS. Lock time varies depending on system fps from 6 seconds at 30fps to 3 seconds at 30fps. This allows you to cover distances larger than what the relative shields would allow you on their own in comparison to ME2.
Besides the accuracy, being out of cover is punished a little less in ME3. You have real dodges that grant DR, and you can use "right hand advantage." In ME2 RHA is frustrating to attempt because Shepard will point his weapon up in the air if you get too close to cover without docking, and so you have to expose more of the character to even attempt to use it.
Enemy composition was also weakened in ME3. Despite the fact that there was now 100% shieldgate for enemies on Insanity, the vast majority of enemies encountered were unprotected. This is in contrast to ME2 Insanity where every enemy is protected and so cannot be CC'd in one step.
Then you have the retooled combo system. Since practically everything makes a combo, and combos do better overall damage than the powers themselves (for the most part), you can basically go through the game CCing unprotected mobs then comboing, then repeating until infinity. In ME2 you either had to spend one of the squad's cooldowns to strip protections first or use weapons fire to strip them before you could get to the CC step. And at that point there was only one AOE combo that was even possible.
ME3 SP Insanity vs MP Gold
Any discussion would need to start with what actually makes for a valid comparison between the two. Many people think that Shepard and squad should be compared to a Gold solo, since there is only one player, but personally I find that comparison absurd. Apples to apples if you care to use the squaddies would be a Gold squad of the player in question and 2 similarly skilled players.
At release, SP and Gold shared practically every stat, from enemy HP to weapon damage. There was fairly large divergence after MP DLC's and balance changes, followed by a little realignment after the last two patches.
In general, the AI is the same between the two modes. However, most Gold units have received HP buffs. This offset by the fact that Gold MP units only have 75% shieldgate compared to 100% in SP.
Weapons in SP and MP are largely balanced the same ever since the last patch, with the exception of SP DLC guns, which are pretty overpowered. However, the depowered MP Venom is still one of the best guns in the mode.
One key point that is often forgotten is that many of the MP DLC characters are pretty drastically superior to Shepard. It doesn't matter that Shepard can have more active powers if he only has 1 shared cooldown to operate them. As a thought experiment, imagine playing SP as a Talon GI with Armored Compartments, Pistol Amp 3, and CM 4. Shepard doesn't get to throw on 100 or 150% shield bonuses at whim, and doesn't usually even have the base shields of many MP characters. His only real advantage is a bit more starting encumbrance (75 regardless of class in SP). Or imagine a Batarian Soldier with Grenade Capacity 5 and thermal clip packs in SP. Shepard isn't really ever as powerful.
Otherwise the main thing Gold has that SP doesn't is more enemies over a set amount of time. And one or two waves have a terrible composition that you never get in SP (like Cerberus 10). So on the whole there are a few waves that are probably worse, and maybe that makes Gold more difficult than SP overall. But it isn't like it is hugely more difficult if you compare them in an honest way, IMO.
Platinum is a different story, predominantly because of the shortened gate durations. If you change SP to platinum gate durations it gets significantly more difficult instantly.