That is true.
Not always (I AM taking into account that you said a "wide range of tasks" not all). Besides, what is "natural" is subjective. Selecting text is something that is better accomplished with a mouse. One's finger obscures the text being selected. Apple's solution to that is pretty innovative but it is required because of touch. To select and copy text using touch requires positioning your finger close to the start of the desired text, examine the magnifying glass to ensure the cursor is located properly, release your finger, then drag the handles to encompass the text, then select "copy". With a mouse, position the cursor, click and drag, right-click "copy".
The point is there are some interactions that are better suited for touch, others for mouse. Some believe it must be all of one or the other. The rest of us prefer to have the flexibility of choosing based on the task at hand.
I don't "need" a mouse for Excel, but from what I've seen/heard from others...
For some it is simply familiarity. They're comfortable and efficient using a mouse. People work differently. For some they'd be better off with a touch interface, for others, a mouse. They like everything else that tablets have to offer but would find a mouse at times to enhance their usability. The response is typically to call them "luddites" and "set in their ways" as if touch is universally superior for everyone in all situations.
For some it is usability. It could depend upon how they interact with spreadsheets. If much of their work includes selecting blocks of cells and copying/pasting them, then a mouse may work better for them. (see above)
Some people forget that for some, an iPad is a tool to assist them in getting things done. Yes, it can show them a different way to do things that might be beneficial for them, but it must also accommodate the way that they approach a task as well.
The addition of support an optional mouse/trackpad to iOS will not impede the use of iOS by those who don't want/need to use one.