@Ben Leybovich and @Joshua Dorkin
As a newbie investor myself, perhaps I'm the rare position of being able to provide value in response to the question that you've posed here.
I myself have repeatedly talked about my professional background in data analysis and financial modeling. My ability to work quickly and accurately in spreadsheets is perhaps my most valuable professional skillset that you can quantify.
I build almost everything out in my own spreadsheets when I plan out my finances, website anlaysis, and yes, my real estate expenses and projections.
That said - as you yourself have noted - I don't know what the hell I'm talking about when it comes to investing... yet. I haven't had ten years investing and learning in the school of hard knocks.
THAT's where the calculators come in. Yes it's data entry. BUT - it's pretty darn easy to forget an input, to not even consider closing costs, to not consider electrical, water, or any other number of expenses that come up in the course of an investment for a newbie. The calculator puts those expenses right in front of your face. It forces you to make a guess on all those numbers, at at the very least acknowledge their existence as costs that landlords must consider - something that the spreadsheet obviously can't do.
For me, if I really want to pull those numbers into Excel and play around with the inputs for hours and run all sorts of fancy monte carlo sims, etc, that's great - I can use the calculator as a guide and do that sort of manipulation myself. I've probably logged 5,000 -10,000 hours in spreadsheets and can model out 30 years in under an hour or so. Most people can't do that. They don't have the time or inclination to develop a skillset that you seem to be taking for granted.
The calculators for less experienced folks serve both the purpose of forcing them to consider each input AND they perform the mathematical calculations needed to predict the future of the investment.
Are they the one perfect solution to anyone and everyone? No. There isn't one of those perfect solutions anywhere. But they are a pretty darn good product to help newbies wrap their heads around basics of a first investment and they provide a framework for intelligently analyzing an investment. Plus - we define each expense and suggest ways to look them up!