[biocit]

English can be manipulated in a way where it has the means to describe almost any sound. "si" is the placeholder for し、just like how し　is the place holder for the sound.

What I'm trying to say is that you can say "Onegai Shimasu" in the same way you can say, "お願いします”。 There's absolutely no difference. Alphabets are just placeholders for sounds.

While you certainly can't produce the same sounds as a native English speaker by writing in katakana, you can definitely produce the same sounds by using romaji to describe kana.