I'm a day late, but welcome to Smashboards.
Thanks for Posting~
I do think the [E]Arts will be useful in both Singles & Doubles. I don't think they'll be utilized better than the Default Monado Arts
(at first) because the timing will be strict, & duration of [E]Arts literally last 6 seconds; that's basically a 1/3rd of Shulk's regular Monado Art's duration. So the proper usage, micro-ing, & the timing to use [E]Arts will be more strict & situational.
When players CP
(counterpick) a different stage that will be a more bigger stage, the [E]Arts will have more space for Shulk to work with, such as [E]Speed being extremely fast on ground & [E]Jump's air speed & jumping ability. If you're planning to use [E]Buster then good luck. . .because even if you'll deal a lot of % damage, the KB
(knockback) will be so petty that you'll get punished early game for throwing out a
spaced Ftilt / Dtilt unless your opponent is a Lightweight. [E]Shield makes Shulk become the sturdiest character with the most weight so stock tanking will be a breeze, however [E]Shield off-stage hinders your recovery so much that you HAVE TO turn [E]Shield off right as you're launched. [E]Smash is just as risky to use as [E]Buster however your attacks dealt will not be punished thanks to the further increased KB, but your % damage dealing is cut by more than half, making a Fthrow usually being 7%, probably going to be dealing 3 - 4% with [E]Smash on.
If you're playing Doubles, & you wanna play a more defensive-based play-style, I'd go with this funny idea I had, SupportShulk!
[D]Arts last for 20 seconds, so, you turn on [D]Speed & you're Sonic zooming around the match punishing stage-long mistakes & setting up throws to your partner for follow-ups. [D]Shield making you a stock tank as it is, 20 seconds is a bliss if you really want to be the ShareStockTank for the team allowing your teammate to be more aggressive.
Ofc [E]Arts can be useful in Doubles too, but the huge difference is the more strict & stressed timing it requires to play [E]Arts effectively. That's all I got.