Personally i like to buy items from uostore when efforts are invested in the correct place. Thus, to thank the dev team and in order to keep the game moving forward.
My thoughts about the extra pet slots:
1. Are not "pay to win" items.
2. You can buy them from players for gold
3. You can make another tamer to get more pet slots, So indirectly, the game is representing alternatives to get where u want to get.
1. The more choices you have with pets makes you a better tamer, so yes they are 'pay to win' if you want to term it like that. No one 'wins' UO so can't see how that applies but it seems it is Pay to enjoy more if you can afford it on top of your subscription.
2 you cannot buy them from other players if you want to use them, as the above said, they should not be vendorable or sellable as people are gonna get ripped off if they buy them as they are account bound.
3. I play siege, one char per account so no, it is not an option to create another tamer unless I open another account. But i would not want to delete other chars just to get pet slots for a tamer anyways even on my prodo shards I play. Stable slots should have been increased for free, or been across all characters on an account, so if I paid for an additional 3 slots they should apply to ALL characters on a particular account, on all shards. So $35 to upgrade the whole account to 21, not just the one char. They are charging a full expansion cost to upgrade ONE fliping character on one shard. Highway robbery.