Buy/ordering many types of fruit trees without specifying which rootstock is used is like going to a restaurant and ordering "food" instead of the specific type of food you want (or need). Rootstock influences:
1. ultimate size of your tree
2. growth rate
3. amount and complexity of maintenance
4. ease of harvest
5. productivity
6. preciosity
7. senescence
8. hardiness
9. adaptation to different soils and soil conditions
10. drought tolerance
11. how well the tree will be anchored
12. whether the tree will need permanent or temporary support (not just how well it's anchored)
13. fruit size
14. resistance to certain diseases
15. resistance to certain pests
16. rootstock and scion graft compatibility
17. tendency to sucker
18. and I'm sure I'm leaving off at least something.
There is no one-size-fits-all perfect rootstock. And, to me, letting the seller dictate the rootstock almost assumes there is. I choose rootstocks based on what is important to me and which rootstock will best address these concerns.