Are you looking more for a weapon or utilitarian device? Also are you saying you already have an axe? Or is it that you are considering an axe or a fixed blade? If it is the later, understand when you need a real 36" (double bit) axe, there is no substitute. If your talking about using a rock / limb to pound on the spine of a fixed blade and do the (light) work of a hatchet - OK, yeah - that can work, but that opens up a discussion on knife manufacturing / steel / edges / and angles.
if your talking about things that are utilitarian but can be used as protection - step back a minute and think about the most likely needs of a utilitarian item. Then consider that some things are more appropriate for a vehicle and not a BOB. Example: a length of chain and a coulple of good shackles are exceptionally handy items, in a pinch you could use it to defend yourself, but what is practical to carry in a vehicle would be dead weight in a BOB.
When your thinking about the utilitarian use - let's be honest here for a second, if you're considering a utilitarian item, that could double as a defensive weapon - for most of us, the odds are far greater that we will use it in utilitarian capacity; not to mention that if one needs a defensive weapon - they really should carry the appropriate tool. Also recognize that some utilitarian tools 'can' do several things - but not well. Some examples (vehicle oriented):
e-tool - will it dig? Sure, but painfully slow. It isn't even close to what a real shovel can accomplish in a 15 minute period. That said, for digging / clearing / breaking up rock, ice / bashing things - I love a mattock
Shop Kobalt 5-lb Hardwood-Handle Steel Cutter Mattock at Lowes.com.
hacksaw - eventually you could cut through cable / chain / fence - but it isn't the same as a large set of bolt cucutters
Knife v Axe
Highlift Jack vs winch
In my rig I carry several things to access and move things, digging is less of a focus.
IMHO - The most practical utilitarian object that can be used as a weapon for either a vehicle or a BOB in this thread was that Stanley Halligan(esk) type tool.