Top 10 man Raiders would abuse the Higher Ilvl's in 25 man to progress in 10 man.
A few 10 mans already farm 25's to funnel BIS to the core 10.
If more rewards are granted to 25's It will create a large disparity between ten man guilds.
Guilds with lots of resources will have a distinct advantage over other 10 mans regardless of skill levels.
This might be different in tier 15 since there is only one instance and no gating (that we know of)
however top 10 mans will still farm 25's during the normal mode week to gain advantages going into heroic's