I think looking at Roserade, that if it wants to actually be good in OU, rather than mediocre, it is limited to certain styles of teams. As aforementioned, it doesn't have the bulk that Celebi does to fit in on a stall team, and Amoonguss also seriously outclasses it on stall, Celebi is a much more potent threat for an offensive team, and Roserade certainly isn't fast enough for Hyper Offense. The main style of team that Roserade could effectively fit onto is a Balanced team. Now Amoonguss can also fit onto Balance teams quite well, however if you already have enough defensively oriented members on a Balance team, but still want the defensive coverage of Amoonguss, you could use Roserade.
There will obviously be players who will be inclined to use Amoonguss over Roserade on a Balance team, even if it means having 4/6 members as more defensive mons, with only two offensively oriented members, however for the general team type I believe Roserade is better. Roserade can provide both offense and support in one moveset, which can be of great value to a Balance team. It would play a role similar to that of the Tinkerbell Celebi set, primarily there for support, however able to fire off seriously powerful attacks when needed. Thats a versatility that Amoonguss could only dream of. Now I know that Amoonguss doesn't really need to be able to bring a lot of offensive firepower, because it has many other methods for dealing with the opposing team, and that is why it is vastly more superior for stall than Roserade. On a Balance team Roserade could run a set along the lines of:
- Leaf Storm/Giga Drain
- Rest
- Aromatherapy
- Spikes/Sleep Powder/Leech Seed/Toxic Spikes
That set would allow it to heal up when necessary, simply by using Rest then switching out, which acts as a Pseudo-Regenerator, granted you need to actually use a move rather than just switching out, but its healing more than 33% when you do it. Aromatherapy would be to alleviate status issues for other team members, which could be quite useful if say one of the offensive members was paralyzed/burned, and it was the only way to break through a wall on your opponents team, or if one of your defensive members was poisoned and really needed to wall something on your opponents team.
Leaf Storm is there to punch holes when needed, and to ensure that Roserade doesn't become Taunt-bait, basically its obligatory on Roserade as it is another feature that sets it apart from Amoonguss. With just 88 SATK EVS and a Modest nature, Roserade's Leaf Storm will OHKO specially defensive Rotom-W 100% of the time, which is becoming increasingly common, and is a pain for Balance teams. Thats just the bare minimum though, you could put 252 EVs into it, taking away from potential bulk, and creating a much more potent offensive threat, even if Grass is a poor offensive type. If Spikes was to be used as the fourth move then Giga Drain would need to be the offensive move for Roserade. Energy Ball could be used, but 5 extra BP isn't worth the healing you miss out on.
The fourth move is there for whatever would support the team more. Sleep Powder is very appealing, as Amoonguss' access to Spore is one of the other key reasons it is used, and I would say without the option to use Sleep Powder, Amoonguss would seriously outclass Roserade. Leech Seed could be used if you wished to give another member of your team a little bit of extra health on a switch, and spikes would be there for that constant extra damage, which is appreciated on a Balance team. Toxic Spikes could be used to get that extra bit of damage going against your opponent, which is once again appreciated by Balance. All up the fourth move allows for Roserade to be flexible depending on exactly what it needs to do for the team.
Celebi has managed to stay in OU, even with its horrible weaknesses to very prevelant offensive threats, due to its versatility and unpredictably, and will continue to do so for the foreseeable future. It can fit in well on almost any team, bar Hyper Offense, and is generally quite a good mon, weaknesses aside. Amoonguss is great for Stall teams as it has very respectable bulk, quite decent defensive typing, access to a 100% accurate sleep move in Spore, and of course Regenerator. While Celebi can be seen as more offensive, and Amoonguss as more defensive, Roserade can sit comfortably in the middle. Its abilities as a Cleric are very valuable to a Balance team, and coupled with its ability to still hit things very hard it makes an excellent member of any Balance team.
So I think that while it only works in a very specialised way, the niche is big enough for it to function effectively in OU.