to some extent this is true about celebi in general - it has a lot of competition as a bulky grass (amoong), bulky psychic (boy there are tons of guys in this department), etc. i have no experience with the sweeper sets; their niche remains i suppose but i'm not sure how good they are right now in the first place.
the real reason to use defensive celebi over, say, latias or amoonguss, is the movepool - perish song and heal bell are incredibly rare and frankly i would kill for them to be more commonly distributed. that fact alone makes celebi worth considering. it is also among the best only mons in OU that learn those moves - toed and gar are the only other two perish singers in OU, and pink blobs/roserade are really the only other options for clerics. with regards to latias, having a grass stab to murder bulky waters right away is nice; natural cure is also a distinguishing factor since it means toxic does not render you useless for the rest of the game.
there's not much else to say about defensive celebi besides those two traits, but they are compelling ones. otherwise it is a fairly typical bulky grass (natural cure is nice; i'd rather have regenerator any day though >_>). invest in sdef and go. i am not a big fan of uturn on defensive sets but alexwolf has pushed hard for twave and yes it is good. celebi is extremely g-weak among bulky grasses so you need a good gameplan - hp fire on the switch is okay but twave is often even better if your team can capitalize on a slow-as-shit genesect.
Celebi and Ferrothorn are the best defensive Grass-types in OU. Celebi does not face a lot of competition with anything else except from Ferrothorn for the position of a bulky Grass type, imo. Of 'course sometimes Roserade or Amoonguss will suit your team better, but Celebi and Ferrothorn are easily the best ones, and for good reason.
Celebi is better than most other Grass-types (excluding Ferro), not only because of its movepool, but also because of its ability, typing, and excellent stats. Celebi's typing is more suited to special walling than Amonguss's or Roserade's, as Ground and Psychic resistances are more valuable than a Bug, Poison, Ghost, and Dark neutrality. Celebi counters some major special threats, such as non Specs Latios, Latias, Sheer Force Landorus, Reuniclus, and CMSubRachi partly thanks to its typing. Even if Amoonguss had Perish Song it wouldn't be able to handle those mons, because Landorus easily 2HKOes, and Latios, Latias, Reuniclus, and CM Jirachi all can destroy it with Psyshock/Psychic. Its ability in combination with Recover, makes it one of the most resilient pokemon in OU. Yeah Amoonguss has Regenerator, but it can't avoid 3HKOes because of the lack of recovery. It is also annoyed by status such as burn, paralyze and sleep, all of which Celebi can take with ease. A burned Amoonguss is always 2HKOed from Specs Keldeo's HP in rain after SR for example, while Celebi is never 2HKOed. Finally Celebi has an actual Speed stat and a real attacking movepool, unlike Amoonguss, meaning it can outspeed and OHKO threats before taking a hit, or take a hit and OHKO with its stronger special attack and secondary STAB move in Psychic.
My favorite Celebi set right now, and Celebi's best set by far in this meta is this:
Celebi @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 240 SDef / 16 Spd
Calm Nature
- Giga Drain / Psychic
- U-turn / Baton Pass
- Perish Song
- Recover
16 Speed EVs to outspeed Adamant Breloom .A STAB move of your choice is obvious. Giga Drain is the all around better move to handle waters, but Psychic is great if you want Celebi to handle better Breloom, Thundurus-T, Conkedlurr, and Gengar. U-turn is used for momentum, and it allows Celebi to not be a free switch in for all the dangerous pokes that lurk in the corner, such as Dragons, Genesect, Scizor, Tornadus-T, and Ttar. Baton Pass does the same, but its main downfall is that it can't break Dnites MS, Zam's sash, and does zero damage to Lati@s, Ttar, and Hydreigon, which U-turn hits quite hard. The price you pay is worth it though, as with Baton Pass you can escape from any non Scarf Tyranitar and Scizor without any worries at all. Perish Song is an awesome move that fits to Celebi's role as a glove, and allows it it beat almost any special set-up sweeper 1 on 1, while also beating last mons and baton pass teams. Recover is used to OHKO Ho-Oh after SR, 1 layer of Spikes, and sandstorm damage :D
EDIT: Fixed the nature thing.