Well, if you have this is as nonmandatory option, then your onlinewallet can be both. You could have one wallet with more flexibilaty and/or one with more safety.
This is desireable, isn't it?
Actually, now that I think again... Suppose you decide to hack into and steal money from an online wallet service. Which is a more likely target: an online wallet with mandatory secure measures, or one without them?
Well that's a trade security vs useability (dunnow if it's worth it or not, maybe).
It could be the standard option - so most users would have it on and only some that dislike it don't have it, if it's mandatory some users might dislike this due to having to send BC two times.
Or there could be other methods offered for those who don't like it (people love having a choice ^^)