Preface
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There was a very long thread [1] on the user mailing list with essentially this question:
"How can I generate a summary report of my expenses only ?" (A similar question would be how to generate a summary report of my income only.)

The thread focussed on the "Income Statement" report and it's lack to hide all references to income (or expenses) and the profit/loss statement.

The Income Statement report has options to
a. select only expense accounts
b. hide the expense headers and totals
c. change the title of the report to be "Expense statement"

What is actually missing is a way to hide the profit/loss statement.


Analysis
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The final conclusion is correct. The current report allows to generate the expense summary as requested, except for the superficial profit/loss statement in the end.

The simple solution would be to add one additional option to show or hide this statement.

However, if the report has to be changed, I'd rather see the options reconsidered a bit further. There are currently two options that return for both the revenue and the expense section ("Label xxx section" and "Include xxx total").
This seems superfluous to me. I can't imagine a report with both revenue and expense sections where you would only want to show a label and total for one section and not the other. I think one would want the complete report to either have labels and/or totals or not, but not per section.

So enhancement number 1:
- merge the two "Label xxx section" options into one "Label sections" option
- merge the two "Include xxx total" options into one "Include totals" option

The fact that is is possible to set these options separately, turned out to be very convenient for the original problem in the list thread, but it can be accomplished more elegantly.

Enhancement number 2:
- When no income accounts are selected, skip the revenue section completely, including potential label and total
- When no expense accounts are selected, skip the expense section completely, including potential label and total

Then finally there is the profit/loss line at the end of the report. This line really only makes sense if there are both revenue and expense sections. If only one section is available, there is no profit or loss to calculate.

Enhancement number 3:
Display the profit/loss line at the end of the report only when both the income and expense sections exist.

Summary
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The three enhancements described above increase the flexibility of the Income Statement report while reducing the number of options to choose from.

With these changes the report can be used 
- as a traditional "Income Statement" report or "Profit&Loss" report
- to get a summary of only income or only expenses

The profit/loss line will only be displayed when it makes sense (ie when both revenue and expenses are available).

The reduction of the duplicate options will make the report easier to configure (without losing functionality).



[1] The thread starts here:
    https://lists.gnucash.org/pipermail/gnucash-user/2010-June/035449.html
    With a related thread here:
    https://lists.gnucash.org/pipermail/gnucash-user/2010-July/035554.html