Font sizes, left/middle/right justifying text, etc... can all be done with the editor toolbar. Realistically, there is no need to allow members access to HTML as virtually all editor functions can be had in the toolbar similarly to MS Word. HTML should only be given to administrators or trusted individuals who in an extremely rare case need to do something elaborate with HTML. Editing in source HTML removes some validation which is done so it is only recommended to allow this for trusted individuals in rare circumstances.
It is worth noting that BBCode (if you allow it in 4.5) will still be processed in Invision Community but upon submitted it will be transcribed in the database just like every other post. Meaning if you go back to edit that post in source, it will not be in BBCode but HTML.