A spell-like ability's advantages over a spell lie primarily in its lack of components. It require none. So while the warlord reborn may already be able to cast the spell transformation repeatedly, being able employ the spell's effect as a spell-like ability, too, can be an advantage in the right circumstances. As gatherer818 points out, some of those circumstances include
while grappled or pinned, or while deafened or emotive blocked, in a zone of silence, with your hands tied behind your back, gagged, hands cut off, tongue cut out, no potions, enemy with a readied action to sunder your potion if you pull it out...
(All of which, I assume, the GM has, at some time or another, inflicted upon gatherer818's hapless characters. My sympathies are with Handless Bob and Tongueless Joe.)
So, instead of casting the spell and all the hassle that goes with that, a warlord reborn merely stares into space for a standard action (provoking attacks of opportunity), and the effect of the spell transformation occurs.
To emphasize futher, likely the most immediately noticeable effect of having the spell transformation as a spell-like ability is that the warlord reborn is spared the expense of the potion of bull's strength that's normally consumed as a material component when the spell is cast.