Because if you're going to do MSU with Necrons (And lets face it- 5 models is a small unit for a t4(5) with a 4+ save) you need to make them both survivable and killy. If you're just going to give them gauss or tesla, just get immortals. Cheaper, better save, can take a transport, and most importantly, comes in tens. The beamer is what sets them apart. No one says you have to take both upgrades, but the point of the 3+ is not that you want them to survive, its that you want the enemy to actually have to try to kill them. 4+ is basically as good as no armor at all in the mechanized metagame, where AP4 or better weapons are all over the place. AP3 is a lot rarer by contrast, and while its not exactly non-existent, forcing the enemy to prioritize AP3 shots at them over something else makes the enemy split his fire inefficiently. What he ignores will kill him.
Ultimately, if you take vanilla blades, you're going to lose them to one round of shooting from a dakka pred or equivalent. They might fire their gauss or tesla weapons once the whole game, whoopdy fuckin doo, for a 20 point model. They don't have the shot saturation to take advantage of the Tesla option, and while gauss is better at just about everything, it, once again, fails to have the shot quantity to successfuly stunlock a vehicle. You'll hit with about three of them, and from there its a 50/50 chance you will get a single six. From there, you have a 1/3 chance to do goddamn nothing at all. Math hammer isn't looking good here. Yes, they get RP, but they can't use it if the whole squad wipes. They aren't meant to be durable, if you want durable, get more Warriors to take advantage of mass RP. They are meant to zip around and kill stuff. Not a whole lot that does that better than a S6 blast on a thirty point model that also happens to be a jetbike that can resurrect. I think a blade with both upgrades, to take it to 40 points a model, makes a pretty good unit that will survive to RP, and then dish some out. If your first move is a turbo boost, 3+ cover, yes, but you do plan to be able to shoot eventually right? 5 Gauss or Tesla shots do not have the capacity to kill an entire enemy unit, plain and simple. 5 S6 blast templates, however, can annihilate a mob of Boyz or Gaunts or IG or anything, really. Worried about bubblewrap around a Lemon? 5 S6 blasts. Green tide? 5 S6 blasts. Gauntspam? 5 S6 blasts. They can even be useful against MEq, what with wounding on 2+. You can drown a tac squad or grey hunters in so many wounds that making armor saves won't help them. For bonus points throw in a stalker to mark enemies for the blades, and give them TL blasts, as well as softening the unit.