I think the trouble with Signal Beam is that Bug is generally just a terrible offensive typing, making it hard to justify the moveslot as coverage. Being resisted by seven different types really reduces a lot of its super effective potential. A neutral Signal Beam does hit a bit harder than a resisted Thunderbolt, but the difference isn't all that big, and there aren't that many relevant Grass types that are actually weak to Bug. I think Ludicolo (which is hit neutrally by Electric) and Serperior are probably the most important ones, and perhaps Celebi for playing on Showdown (no idea if it's in the same tier as Xurkitree, I'm very unfamiliar with Showdown). But if you look at other commonly seen Grass types, like Venusaur, Tapu Bulu, Decidueye (it's not that good but still quite popular because it's an Alolan starter), Kartana, Ferrothorn, Whimsicott, Breloom, they're all hit neutrally by Bug. Mega Sceptile is also weak to Bug while being immune to Electric, but is hit harder by Dazzling Gleam.I think Signal Beam deserves a mention, since it hits Grass-types harder than Thunderbolt does.