The whole sidearm thing is very situational, and honestly, it only works for those heavy str builds. An agi build pike/longspear can just helicopter once and run off. Or block and run off. You are support, you shouldn't be 1v1. Also, learning your weapon really helps. Last night you tried to 1v1 me in the midst of siege Caita. You dropped your pike and rushed at me with a langes messer I think? I poked you down as a hoplite, no sidearm, no dropping of shield.
This is true, but I've also killed both you and Relit multiple times by switching to my sidearm as well as by using my Pike. Killing someone a few times doesn't mean one way or the other is better. (Though I really wasn't paying much attention last night in siege
)
Helicoptering once and running off is fine and good unless that's a target that NEEDS to die, or if you're in a situation where you can't run.
Example: Thrower you've managed to get close to, and has an ally on the way. You've been separated from your allies and they won't get to you before the thrower's ally arrives. Thrower is just holding downblock, so pike/LS is useless.
Without a sidearm your only choices here are to run, at which point you get shot in the back, or fight a 2v1 fight. With a sidearm, you can do a rapid switch, spam the thrower to death, and then not be in a 2v1 when his ally arrives.
If I don't have a sidearm and my allies aren't around/are occupied/are dead then I'm at a serious disadvantage against players that know how to fight a pike. Pike/LS 1v1 is all about deception and trickery. If someone knows your tricks and has experience fighting them, you're not likely to win. Also, if I'm simply in an area where I can't helicopter or run away (such as on a steep incline), having a sidearm is exceedingly useful.
It also means I'm able to operate on my own if need be without fear of getting stuck in a hopeless situation. For example, pulling a 2 pronged flanking maneuver with 2 people. With your setup this kind of maneuver is very risky if you run into someone you can't kill very rapidly with the longspear. With a sidearm you can swap it out, kill the target, and move on.
Besides, my langes messer is only 1.7 weight, and it's one of the heavier sidearms. That amount of weight isn't noticeable, and for the massive utility it adds I'd say it's a hundred times worth it. Admittedly, using a sidearm requires some more practice (especially practicing WHEN and HOW to switch to it), but there are simply too many situations where it comes in handy.