Because a VOIP call using 3G whilst roaming would be cheaper...?
This is a misnomer - when do you actually use Skype for voice? For me, it's when I'm travelling and want to make cheap calls home, rather than paying exorbitant roaming fees; I make those calls either from my hotel or a local café, to make use of free wi-fi - it would be even more expensive to make a VOIP call over data when roaming than it would be to make a standard voice call. The only other time I would be sat in front of my laptop anyway, and could make the call that way (looking handsome in a headset - at least its in private). Skype to Skype calls would be the same, and I'm not expecting to completely replace all of my telephony requirements with a single iPhone app anyway.
However, _if_ you didn't pay for the data usage, and only paid the call cost, and _if_ that was at the same rate that Skype charges for calls to phones (landlines or mobiles) then yes, I would use that whilst travelling - as it would be a lot cheaper than making the mobile call (and often quality is better as well). However, I don't see Skype having a relationship with every MNO to enable a revenue share arrangement, and if they did it's unlikely to be at their standard pricing levels.