Plenty of hosts now use mbstring and such, and in time likely most hosts will have it.
Its meant to be a CMS for a newer generation, providing better options, but yest require more modern hosts.
It gives much better options for people of different languages, to be able to use utf8 across the board correctly.
For english which i'm guessing you mostly deal with english customers, your right they will never need it likely.
And I could be wrong but I don't think you have to completely compile PHP with mbstring, you only need to enable the extension.(never had the need to compile PHP myself)