While I see your point, when I think about the Underground Hallow piece, it's basically a combination of the Hallow and Underground music pieces. Why not do that for all the songs aside from the night ones?
Underground desert uses guitar, so I thought "why not?"
For me, guitar is instrument that shows something dangerous, chaotic. Corruption and crimson are very dangerous biomes, especially for pre-hardmode!
Um, yes, this is a valid argument, however, in my opinion Underground Desert's music is a bit bad as it lost some of the original style of the game that it had at the beginning. The first songs made for Terraria were genius, others that came later were just good, and those from version 1.4 left me a little disappointed, especially the one I mentioned (Underground Desert). So I think that Nighttime Crimson & Corruption shouldn't have an electric guitar, if you did a kind of mix of the original songs (Daytime, Underground & Forest Night), being the first two respectively from each biome, I think it might sound cool.