If they are already gassed from fighting somebody else, and you're going in to poach, you're striking at undefended worlds. Maybe one defensive ship, some transports and colony ships, maybe one newly produced ship if your timing is bad. For a hair more than the price of an axe, you could have a battle cruiser, with actual attack value.
True, I could build a battle cruiser in most cases instead of an axe. However, I don't like the looks on paper of a battlecruiser, and have never bothered to build them. The only good thing about them is the attack value; everything else sucks. A mere 20 HP on a 2nd tier offensive vessel, what's up with that? And for 5 maintenance a pop? No thanks. I view battlecuisers as worse cutouts than axes, for a higher cost. Plus, I generally don't need them. If I'm attacking with my axes, it's mostly non-combat stuff or 1st tier crap units; if I think I'll need a real attacker, I'll build a hammer or two.
While the game may not pick the best defender first, AFAIK it is supposed to hit combat ships (with ATT>0) before non-combat ships (with ATT 0) -- thus, the axes are there to (a) make sure the transports don't get hit if my stack gets attacked, (b) free up my hammers (if any) from needing to stay with the transport stack to protect it, (c) take care of any weak defenders & non-combat ships that happen to be at the target planet, and (d) not die while doing so, so they can defend my new planet if necessary. Could I do the same with other 1st and 2nd tier units like corvettes and battlecruisers? Probably not, given the rest are all horrible at defense & die too easily. The battleaxes may be able to support real warships by taking over some of the more defensive roles, but other cutouts can't. Since none of the other cutouts are really useful poaching-wise, that leaves battleaxes as the cutout of choice.
I should note that I don't build extra axes explicitly for the purpose of poaching. These axes are just some of my cutout defense fleet that get drafted into poaching duty. I should also note that to date I haven't built many cutouts with the express intention of scouting with them; I find that on smaller maps, scouting with colony ships, the explorer, maybe a scout, and a couple free ships out of anomalies usually suffices for initial exploring, with sensor drones coming later in more of a global coverage role.
EDIT: A couple more amplifications...
But I also tend to play larger maps than you do on average. The smaller the map, the faster you get into conflicts, the easier it is to hit targets of opportunity of all shapes and sizes, including squishing minors, poaching red meat from AI-on-AI wars, deciding to launch an early invasion, etc.
I agree that early game poaching is easier on smaller maps than on larger maps, as both range tech / starbases and transit time are significant factors. Even on medium maps you really need Grav Accelerators to make battleaxes viable as anything other than pure home defense. Corvettes might well have more value on larger maps, where scouting takes longer and you need faster ships generally. I still wouldn't build battlecruisers though.
However, even on small map conquests, I usually rush to hammers and rely on them.
For real conquest, I don't use axes much either, though I may bring along a couple just to be warm bodies. I fully agree that for attacking an opponent who can actually fight back in a meaningful manner, you need at least hammers.
Right? 
