Hi,
If you bypass the check and allow the guest to be seen as a logged in user (so by just changing a line of code basically), anyone using an address email of a registered user on your checkout will have access to the addresses of the user, will be able to use his points, etc. That's why we're talking about security issues with that.
To avoid that, it would require differentiating guests and registered users in HikaShop, but that's hard to do because we have a unique key on the user_email column of the hikashop_user table (like Joomla has in the users table) so that you can only have one entry per email address.
The advantage of doing that is that we can easily synchronize the user entries between Joomla and HikaShop and other optimizations. And the code of HikaShop supposes that you can have only one entry for one email address.
So changing that might have implications in places I didn't think about, like the votes, etc.
Basically, it looks like a simple change, but it has great implications if we want to do it properly.
For anyone else interested, I've replied with informations on where to change the code to allow for that:
www.hikashop.com/forum/checkout/892402-a...ed-users.html#280675
Note that as a customization in your case, you don't need to differentiate the users between guests and registered users, so you probably won't have to do all that. This was to explain why we can't add that capability easily in HikaShop so that it works correctly for everyone.