The thing is, when you buy an N-Gacha, you pay 1500 and ultimately get an N wild. That's a net loss of 500 pero. It's also time consuming to roll THAT many N-gachas. Plus, if you have every SR card in the N-Gacha, sure. You might get an R card or an SR card, but it'll add one point to a random card. Even if it's an SR, if it's a low level Sexy card, your SED boost is non-existent.
Ultimately, it's worthwhile to have a mix of both. I stock up on N-Gacha during gambit because you get a pink key with every ticket. The pink keys are typically worth either 1000 Pero, 3000 pero, or a nice consumable. If it's not Gambit, buying N Gacha just isn't as pero efficient.
Plus, every event, you will be spending anywhere from 1-50 rolls to get the new N-Slayer. Sometimes you get her on the first spin, and sometimes you chase her for all goddamn day since she pops up in EVERY box but the one you click. Usually, you will get the N slayer within 5-10 rolls, but those are the chances to burn a bunch of N-wilds. Plus they're useful for doing the mission to get three N cards and 2 R cards. You can reliably do those in a few rolls per day.
But when it comes to events like EG, you may need SED and need it NOW. Sometimes you just don't want to wait for your SED to grow slowly. I had a shit ton of SR wilds that had been sitting around accumulating, and I burned all of them (holding 100 in reserve) to boost my SED by about 100k. I also burned all but 100 of my R wilds to make my base SED as high as possible.
I don't advocate ALWAYS buying N-Wilds for the conversion to R-Wild, but if you have pero to spare and a good supply of N-Gacha, it's a relatively easy way to add a few thousand to your SED. Sometimes a few thousand SED makes a difference, especially if you manage to get an R or SR slayer.