They do not all need to be equally powerful, equally popular, or equally successful. But they do all need to be viable, and they need to be different.
Making a sweeper is easy. Making a sweeper who is also a viable screener or sporeshuffler or dual screener - with the same stats - is hard and would make us look at the pokemon already performing these roles, then tailor CaP2 to fill the niches that remain.
Conclusion - bulky offense is indeed a nice sensible 'main role' for CaP2. But let's not close our mind to the beauty of this concept in the first place... we can only meet its vision (and its potential) if we allow some other movesets to be used on a frequent basis as well.
You basically listed reasons for why bulky offense is the (arguably) the best set for CAP2 to have. With choosing a concept of Sketch Artist, we as a community are basically asking for
all three of the roles Dusk inquired about to be fulfilled and explored. I'll try not to poll jump with this post, but I'd like to make some comparisons to other Pokemon in the 5th Generation metagame that advocate for why bulky offense can suit a variety of roles (namely Supportive, Offensive, and Defensive).
Offensive
I provided the example of Scrafty on the first page of this topic, and that opinion still stands. Scrafty's speed doesn't allow it to outright sweep, yet it can boost it enough to pose a threat. It's bulk allows it to take a few hits while setting up for a sweep. CAP2 could explore a similar variety of set-up boosters. Furthermore, it would discourage Shell Smash, because with the drop in precious defensive stats, it could easily be taken out with priority.
On the other end of the offensive spectrum that I'd like to introduce is Rotom-W. Again, its speed is comparable to Scrafty's: slow, but still usable. Where Rotom-W differs is its access to fantastic STAB moves like Hydro Pump and Thunder. Furthermore, its considerable bulk allows it to provide support with moves like Will-O-Wisp and Trick. What CAP2 can learn from Rotom-W is that access to a ridiculously strong Base Power move (perhaps even STAB'd) is beneficial as well, which would further increase the range of moves utilized by Sketch.
Defensive
Hmm, defense certainly isn't as viable in OU anymore, in my opinion. The easiest comparison that comes to mind is Bronzong, who is capable of taking many hits thanks to its key resistances and immunities. Like many bulky offensive Pokemon, Bronzong also has usable defensive stats that allow it to take hits, even with a lack of stable recovery. CAP2 with the goal of bulky offense could take advantage of its bulk to wall certain portions of the metagame. Heatran is also a key defensive Pokemon that has naturally great defenses. CAP2 with a bulky mindset would be able to utilize its defenses with moves like Recover or even Aqua Ring + Protect. Also, what makes many defensive Pokemon shine in today's metagame is their great movepools: Heatran gets plenty of toys, Blissey can pass gigantic Wishes, and Vaporeon gets Rest + Hydration. Perhaps if we want to make defensive strategies more viable on CAP2, we'll have to rely on movepool more than stats to accomplish that goal.
Supportive
A bulky offensive Sketchmon could utilize support as well. Plausible defenses would allow it to take a variety of neutral hits while providing support with Spikes, Spore, or what have yee. Furthermore, a moderate amount of speed could help it put an end to slower defensive/support Pokemon. Gliscor and Deoxys-D come to mind for me personally. With their modest 95 base attack and workable defenses, they're able to break support while providing their own (one less so than the other). Considerable bulk would allow CAP2 to also set up multiple layers of hazards; think Ferrothorn but with (probably) less resistances. Like my defensive reasoning above, movepool other than Sketch will also play a large role in how well this Pokemon can perform defensively.
So in conclusion, I would like to say that
bulky offense would allow CAP2 to
host a variety of roles for CAP2 to fill. I think most everyone agrees that they want Sketch to have as wide a range of a viable movepool as possible (who wants Shell Smash and Spore to be the only moves used). I'm here to debate that bulky offense would allow us to tap into the potential to the greatest degree. I hope I didn't poll jump too much with this post, but I wanted to make some comparisons that don't seem to clicking within this discussion. By choosing a concept with Sketch, we are basically asking for a Pokemon that can fill all kinds of roles, from sweeping potential to supporting skill. Bulky offense would allow us to explore all of those roles significantly. Thanks for reading =D!