Sharding eliminates server load and realm stability. Cross-realm sharding is here so that players are evenly distributed across realms and helps combat underpopulation. Sharding at launch is vital to ensure a stable launch of an expansion. Starting zones would be filled with millions of people and that would turn even accepting a quest into a nightmare. Disadvantages of Sharding
Only really to expand on Gráinne's comment as regards the 'collecting' and 'roster development' sections - which are, as said 'distinct' although they are intrinsically linked - I would like to point you in the direction of the in-game Achievements > Battle Pets > Collect tab. There are a series of achievements called Raiding With Leashes, pretty much all of which can be completed via the auction house - if you have the gold/inclination - or by completing old raids.