"OK, here is some random bank's ATM which charged me $44 to get $40 (2 x $20) out, and now you say you don't have $12.5 in change on you ... well, shit." ... "What will you prefer: GWallet or FB ?"
"Oh you need the full $27.50 because your life will collapse due to a lack of $7.50 in cash although nobody uses cash, well, there's an ATM in the lobby thats free on my network, I'll spot you $30 on the $27.50 and you owe me a can of soda next time"
The problem with a bazzilion services all trying to extort small transactional fees either short term or long term is the marketing about their fee can't avoid mentioning that the fee is really small and we're discussing fairly small amounts of money.
I am okay with rounding off amounts with my close friends. But with other people there is that social angle with being comfortable enough when I tell them I will buy you soda next time. Plus consider this "buying soda next time" with 10 people at the same time in either direction. The question is not about $2.50 being a small amount. I like being exact about money and so do lot of my friends. May be I am from a different background where $2.50 is still money that should be cleared.
Sidenote: Usually such fees are O(1), i.e., independent of the actual amount being withdrawn (and even applies to non-monetary transactions, such as balance inquiry).
But maybe I'm cynical because I'm in Austin and driving anywhere is just miserable.
I'm not going to go home and remember to paypal on my desktop when I could just use the venmo app to send you money right after you paid.