There has been a topic about how the engies/hotwires/technicians are all around characters a long-long time ago. They can repair buidings, tanks, infantry, highly efficient in combat with remotes, etc and the OP came up with some "balance" ideas. Afair there were some good ones, but the majority disliked it too much and it was scrapped.
Why does this come up now?
Before jumping on conclusions I'd recommend you to find the root of this "problem".
I call an infiltrator a player, who can successfully sneak into a base and cause havoc, by blowing up buildings, stealing unmanned vehicles, killing unsuspecting units, etc. Now, most of the players are not capable of this, even the guys who call themselves generally experts are pretty bad at this in my experience. On the other hand there are a handful of players who are specialized/highly efficient at this, and most of the time they cause the real disturbances which are remarkable and well remembered.
Although SBHs can enter enemy bases, but with a very little effort compared to visible units. Personally I don't call SBHs real infiltrators, because it's a "cheap" way to get in, no wonder why SBHs are noob-magnets.
What does it have to do with sidearms?
Honestly I don't know what the fuss about it. I do understand if you want to prohibit the cheese unit (SBH) to get his hands on powerful upgrades for various reasons, although others might not agree on this with us. But keep in mind that new players to RenegadeX prefer the SBH because it's powerful and easily give them a sense of achievement. Ripping him off from the "good weapons" would make it less appealing. This might be good for us, but not for the game.
What about the Hotwires/Technicians as infiltrators with sidearms?
For example if I don't "ghost" on Walls I have to deal with 3 infantry units on average until I get in a position to enter a building. By then I'm usually injured and low on/out of ammo. Does the extra sidearm helps me on this? Most certainly, but don't blame the weapon because I blew up something, blame those guys I killed who didn't tell the team about my coming or the team which failed to respond to this. And we all know it doesn't matter what you bring when there is dedicated, good base defense.
Anyways, with the mindset of taking away the extra sidearms from SBHs, Eng/Hot/Tech units, we could easily take them away from snipers, because they are clearly long range units and it's quite embarrassing when you ambush a sniper at close range and suddenly you face against an effective close range weapon. This could be applied to many units, but there is no point carry on with this. Also, it has been stated many times, that buffing a unit in any way are always benefit better the good players.
One more thought about this, because I already wrote a wall of text.
I don't want to scarify B4 don't take me wrong. But it played a minor role (compared to real life issues) that I quit playing less then a week after it's release, because it made infiltration -destroying buildings- totally pointless. I believe we don't have to go into the details.
The main reason you find yourself buying powerful sidearms constantly, because you can afford this with the current broken economic system. If you recall it, you could't afford to spend 700 creds for a highly risky infiltration attempt over and over in B3, not to talk about a fully armed hotwire with a tank.
Hope I stayed objective on this matter, so my conclusion is that I found this a bad, half-baked idea.