Unsolicited redesigns are a great way for designers to hone their skills, and feedback from the original designer is often a good way of explicating the other design decisions and trade-offs in creating the design. Sometimes they lead to valuable insights, while other times they devolve into link-bait arguments. Similarly, design commentary is a valuable source of information as to the usability of a product.
There are valuable design insights to be gleaned from these sources. Seeing a different solution to a problem can spark new insights and new designs. Commentary can also spark useful debates about particular approaches and aesthetics.
That said, visual design is only an aspect of software design, not its entirety. UX designers also have to help make decisions about affordances, functionality, trade-offs, business decisions, and monetisation.
Unsolicited redesigns are often built from a simple understanding of the business problem. The designer may have no knowledge of the development restraints and requirements, the internal negotiations, the legislative requirements, and so on. At best, a redesigner can interpret a design within some best-practice guidelines and a visual design aesthetic.
Many visual designers and commentators default to a fairly minimalist design aesthetic, drawing on influences such as Bauhaus. While there's nothing wrong with having a particular design aesthetic, these values can conflict with the requirements of effective UX design. While skeuomorphism might appear ugly to designers who appreciate tasteful design, it can be very useful for interaction framing and storytelling.
The popularity of UX and user-centred design has led many visual designers and developers to try to adopt UX principles and best practices. There are numerous UX blogs with easy-to-follow guidelines and checklists that give you a sense of what a user-centred design approach could be. But being interested in UX and having an appreciation for UX does not mean that you are doing UX.
UX, visual design and front-end development are all related fields, and practitioners must be able to work together productively. But it is a mistake to presume that this collaboration doesn't happen without arguments and trade-offs.
Software design is always about working within constraints, affordances, and restrictions. Prescribing a design approach that prioritises a visual aesthetic over functionality is the opposite of user experience.
Barry Saunders is a UX Architect. He has worked on projects for News Ltd, Google, Westpac, EFIC, WWF-Australia, QUT, and SBS.