Everyone else will have far more strategic, immediate, and convenient information than you will regarding wait times, sudden closures, available dining. You won't receive alerts if a sudden closure affects one of your Fastpasses. You won't be able to review your Fastpass selections at will at all, unless you're standing in front of one of the limited number of kiosks. You won't be able to plan anything on the fly, or in park two while you're in park one. You can do it, but you'll be experiencing WDW from five years ago while everyone around you is experiencing WDW of 2017. The whole "you can do it if you want to" thing does not mean you'll have an equal, or even a slightly lesser experience. It just means WDW can't make you buy a smart phone. But make no mistake, the New Tech system (
MDE, FP+, MBs) is aimed squarely at smart phones. You
will be negatively impacted.
If you can afford WDW, you can afford upgrading your phone. You should.