In summary: Whatever the source (rule or explicit), Breeding Pool needs 2 mana abilities because other cards can take away one basic land type (and thus mana ability) while leaving the other intact - 305.6 simplifies the templating necessary to add and especially take away mana abilities based on the basic land types a land gains or loses
owyn, Fearless Knight owyn, Fearless Knight is a solid commander that can effectively clear the board of bigger creatures
10 Final Fantasy 4 - 20 Cards Daredevil Cid Leads The Pack Final Fantasy 4's 20 cards are, naturally, inclusive of legendary creatures for Cecil, Rosa, and Kain
Now that we have more information, however, it appears things go even further than that
Tellah, Great Sage A useful way to evaluate cards is to consider what similar effects cost, and whether you get a discount on the ability