As SBB said, it's not Ghost-type in general you have to worry about, it's just Aegislash. Gengar is the only other really good offensive Ghost in OU and due to its not so great power it's not so hard to check even without resists to Shadow Ball, with specially bulky Pokemon, or just by taking advantage of its frail physical side and nuking it with priority (many offensive teams carry up to three priority suers, so Gengar definitely has a difficult time doing much against those).
Packing a Ghost-type resist to deter Aegislash from freely spamming Shadow Ball is nice, but not a must at all, especially on offensive teams. Between Rotom-W, Lando-T, Excadrill, Bisharp, and Garchomp, which are all over the place, Aegislash is checked pretty well. You must always be careful not to give it too many switch-ins with Lati@s and other stuff that are free switches for it, but in general it's pretty easy, or rather manageable, to keep Aegislash in check even without a Ghost-resist.
That said, Ghost-resists or better, Aegislash counters or very strong checks, are a must on any team that has a defensive core and doesn't plan on finishing the game in less than 15-20 turns, aka balanced, stall, and even some bulky offense teams. Be it Mandibuzz, specially defensive Hippowdon, Krookodile, or Chesnaught, you gotta have this thing covered. So either pack a strong check/counter, or multiple checks (for example, Heatran + Rotom-W + Lando-T) if your want your balanced/defensive team to have any success on the ladder.