I wouldn’t say it could be used as a “sort of druid shape”, when the two magics are innately different. Druids get their magic from nature, or spirits of nature, and they train specifically with the skill. This gives them total control over their forms, and allows them to utilize the instincts and abilities of the animals they transform into - without the room for error commonly known in arcane transformations.
However, I would argue and agree that there can sometimes be benefits to arcane transmutation, as seen with Medivh, who transforms himself into a raven to travel swiftly across great distances. Medivh is a particularly skilled practitioner of the arcane arts, and might not be the ideal standard for all mages and generic arcane transmutation magic. The above books on Polymorph Rules & Regulations also alludes to pitfalls with polymorphing that do not transfer to druidic nature transformations, such as:
- Cerebral brain function rarely translates in a polymorph… But sometimes it does [This is based on chance, and implies that intelligent creatures turned into less-intelligent creatures (or objects) typically lose intelligence or vice-versa]
- Polymorphing into creatures like oozes, ethereals, silithid, and worst of all dragonkin is severely punishable… [followed by, see below]
- Accidents happen, and the Kirin’Tor understands this. However public viewing of creatures like the Were-Whale, Mur-Elf, Amphibious Worgen, Flying Ooze, Grypho-Hippo-Wyvern, Qiraji Bunny, and the infamous “Blue-Checkered Cube” give us all a bad reputation. [This passage suggests that more difficult creature shapes are punishing, and can lead to disaster and possibly death]
- Do not polymorph inanimate objects! - We can’t believe the Kirin’Tor is receiving reports of this, but such a heinous act caused the development of the brand new Highly Severe Claims Disaster… [This is yet another implication of original intelligence not being transferable when polymorphing]
So, where druids retain their personality and intelligence when transforming, it’s not the same for polymorphing. If a human is turned into a cat, it suddenly has the intelligence of said cat. Of course, the “benefit” of polymorph may come from risking more dangerous shapes such as a dragonkin (which are vastly intelligent creatures). But according to the references of this in the lore, it seems more likely that luck would not be on the side of the arcanist attempting to do such.