As a developer how do you guys deal with being asked to “assist” for a while with tasks that aren’t really software development?

It all depends. For the most part, as a developer you should focus on development. This is because:

• That's your contractual responsibility
• It's the reason why your employer hired you and what they want you to do
• It's what you want to do (otherwise you wouldn't be a developer)

But it's perfectly fine to assist here and there. At the end of the day, it's good to provide maximum value to your employer. Sometimes that means dropping your duties and doing something more urgent (such as assisting with testing or with product management).

A few things are at play here:

• If you're happy to do those other duties, then go for it whenever necessary. As you suggested, it can be a good learning experience.
• It's less efficient for you to be doing non-development stuff. After all, you're not a specialist at those other duties like other people, so you'll take longer to do them or you might not have as much expertise as them. For this reason, you should bring up these concerns so they're fixed. For example, if it's common that the testers in your company can't keep up with the work, then the company should fix it in various ways such as hiring more testers or changing the structures of the teams to be more balanced (more testers or less developers in each team).
• If you don't want to do those other duties and you're not legally (contractually) obligated to, then maybe you can avoid them. It's technically the company's problem, not yours. Consider doing something like this: Do them only when absolutely necessary. Urgently speak to your managers about how they should handle the situation immediately because the company is losing money and efficiency. If it's not urgent, say you won't do them but provide reasonable excuses (otherwise you might appear uncooperative). "Sorry but for this week I have been assigned to do tasks X and Y and I don't have the time to assist on task Z."