Lucario is forced to used either a 80 or 60 BP attack with terrible type coverage to hit everything faster than it. Garchomp already outspeeds a huge number of pokemon in the game (I have the exact percentage somewhere, I'll find it) and the number of pokemon that outspeed Garchomp compared to the number of pokemon that outspeed Lucario is a pretty big number.
It's not really fair to say Lucario is "forced" into using it. It's an advantage that he has the option to use it. In other words, exactly what umbarsc said. :/ Yes, Lucario needs to use Extremespeed to beat Electivire or Tentacruel. On the other hand, it can also use Extremespeed to beat Weavile, Starmie, anything Scarfed. Garchomp doesn't have that option at all. Lucario is the one better off here, not Garchomp. On a related tangent, Ice Shard is much more common than Mach Punch -- not only does Lucario have access to his own priority attack to fight off slower priority users attempting to beat him, but it resists the most common one anyway.
The number of
significant pokémon that Garchomp outspeeds and Lucario doesn't really isn't that large. (The raw number is pretty high though, mainly due to the bloated list of Base 100's in BL.) Here's the list of OU and BL between 90 and 102 Base Speed inclusive:
Garchomp, obviously. Sadly, tying other Garchomps is largely the single best reason Garchomp needs its own Speed.
Celebi and
Jirachi? I'm about the only person in the world that typically runs 300+ on them, the vast majority are bulky sets with little to no Speed at all.
Zapdos. Can be faster but typically runs bulky on one side or BP sets. Usually has a +SpA nature to take advantage of its much higher stat there.
Salamence. None of its most common movesets are +Speed. Specsmence is almost always Modest, Mixmence can't afford the loss of attack stats, and DDMence rarely goes past 270. Annoying to physical Lucario for its Intimidate and Fighting resistance, not Speed.
Tentacruel. Probably the most common OU speedster that would be faster, especially SD and Specs sets. Also a common user of Toxic Spikes, which Garchomp hates but Lucario is immune to, and the most susceptible [of the 100-base OUs] to Lucario's Extremespeed.
Yanmega. Speed Boost allows it beat Garchomp anyway, let alone Lucario. Susceptible to Extremespeed, double-weak to SR, and can't touch Lucario if it lacks Air Slash.
Electivire. Unless things have changed recently, it's usually +Attack. Will also outspeed Garchomp if it absorbed an Electric attack. Susceptible to Extremespeed.
Gliscor. Never maxes Speed. Gives both Garchomp and Lucario trouble due to its high Defense and (BAN ME PLEASE) Sand Veil.
Roserade. Often max Speed for Specs or Subseeding, and thus ties Lucario. Can use Toxic Spikes. Susceptible to Extremespeed and does limited damage anyway if it lacks HP Fire.
Lucario. Duh. 50/50 shot at the kill like Garchomp dittos, fair enough.
-----BL-----
Charizard. Charizard is only BL because it's fucking CHARIZARD, it's not really that good and you don't ever see it in top teams. That said, will usually be faster than Lucario but lose to Extremespeed after SR damage.
Shaymin. Hell if I know, but I assume it probably runs fast, unlike Celebi/Jirachi, since it lacks 50% recovery. Lacks damage without HP Fire but is resilient to Extremespeed and a Specs set will blow Lucario away with Seed Flare anyway.
Flygon. May or may not run faster and resilience enough to eat Extremespeed at high health. Garchomp +1.
Staraptor. Can blow up Lucario if it's the more common Jolly, but it's suceptible to Extremespeed.
Entei, Regigigigigigigigas, Typhlosion. Typhlosion is only BL because it eats UU alive. (If it ever showed up in OU it would be faster than Lucario, albeit somewhat weak to ES.) Entei and Regigigas, lol.
Slaking. Scarf will outspeed both. CB is a solid victory for Outrage Garchomp; Earthquake won't OHKO.
Arcanine. Sometimes opts to be slower due to having its own Extremespeed. Some will be faster, though, and its high HP plus a possible Intimidate lets it tank Lucario's ES fairly well.
Houndoom. Faster but susceptible to Extremespeed.
Uxie. See Entei and Regigigas. The few in use are bulky, which Lucario may OHKO and Garchomp won't.
Zangoose. Ties Lucario and has its own Close Combat. Susceptible to Extremespeed.
Moltres. Like Zapdos, is often Modest to take advantage of its higher stat. Goes 50/50 with Stone Edge and beats Crunch/ES Lucario, but it suffers Charizard syndrome and loses to SR + ES automatically.
Porygon-Z. Ties Lucario, may be Scarfed. Susceptible to Extremespeed and can hardly touch Lucario anyway; Garchomp gets punched in the mouth by Scarfed Ice Beam.
Ones where Garchomp has a significant advantage
due to Speed are bolded. Aside Lucario/Garchomp themselves, the likes of Flygon and friends aren't too common. The only other remotely common offender is Tentacruel, whose Toxic Spikes are liable to impede Garchomp anyway and only deals significant damage with Specs. Shaymin and Slaking are also only OHKOed by Outrage, so Dragon Claw Garchomps still fails despite its Speed.
In other news, fuck Roserade and Venusaur is way too underused for its own good.
You do realize that Bronzong is the 9th used pokemon overall, with Skamory not far back at number 22, right? How can you say they are "not common"?
Gengar alone is the second most-used pokémon behind Garchomp and beats Bronzong + Skarmory's combined usage figures. Deoxys-E is #6, Infernape and Starmie are at 12 and 13, Azelf at #16, Weavile at #19, and #21 Heracross is commonly Scarfed. (Heatran at #5 is also commonly Scarfed, but laughs at both ES and BPunch anyway.)
Are you really saying it wouldn't be worthwhile to use Extremespeed over Fire Fang, if Garchomp had that option? Gengar is immune to it anyway but the list of pokémon ES would see use against is still much longer than "Bronzong and Skarmory." Forretress as well, to be fair, but Forretress is liable to ruin Garchomp via Toxic Spikes anyway and is countered by Magnezone just like the other two.
Lucario
does have the option. The exact same reasoning can be used to justify Dragon Dance in comparison to Swords Dance as well, albeit on a lesser scale. (A second DD is required for the likes of Yanmega/Ninjask, Deoxys-E, really fast Scarfers and one DD loses in power what it gains in Speed.)