there are a few problems with the jump between 10 and 25.
setting up the scenario you have 25 able people of equal skill, 10 of which can be made into a 10man.
firstly the gear goes
10man
10man hc
25 man
25man hc
however the difficulty is generally
10 man
25man
10man hc
25man hc
this means the incentive of doing the 10mans is there because of the gear being equal to a 25man on a hard mode
also the 10mans arent balanced around having a shaman so heroism + 25man gear is equal to you having the same lvl gear as if you already completed the instance