Edit: I forgot, he can't hit sixers and fours like UVVVVVV can, so nevermind, he can rot forever at the lower grades of cricket, Yuvraj will be there to save with his tons after 2/3 series' of failure.
Here are Yuvi's last 10 series':
1. vs Srl in Sri Lanka (2008): 72 @ 14.40, HS of 23
FAIL
2. vs Pak, Ban, Srl in Sri Lanka (2008): 157 @ 52.33, HS of 48
SUCCEED
3. vs Pak, Ban in Bangladesh (2008): 137 @ 45.66, HS of 56, two 50s vs. Pak
SUCCEED
4. vs Aus, Srl in Australia (2008): 202 @ 22.44, HS of 76
FAIL
5. vs Pak in India (2007): 272 @ 68.00, HS of 77, four 50s in 5 games
SUCCEED
6. vs Aus in India (2007): 192 @ 32.00, HS of 121
BARELY FAIL
7. vs Eng in England (2007): 283 @ 40.42, HS of 72
SUCCEED
8. vs Rsa in Ireland (2007): 123 @ 123.00, HS of 61*
SUCCEED
9. vs Ban in Bangladesh (2007): 25 @ 12.50, HS of 24
FAIL
10. vs Ban, Ber, Srl in West Indies (2007): 136 @ 45.33, HS of 83
SUCCEED
I'll include the previous one, too, since the performance in the World Cup was largely against lesser teams:
11. vs Srl in India (2007): 116 @ 116.00, HS of 95*
SUCCEED
As you can see here, in his last 11 series, Yuvraj hasn't really been abysmal in 2 back-to-back series'. I think you are paying attention to much to the fact that he got dominated by Mendis. I think apart from Gambhir and Dhoni, he's been one of our more consistent ODI batsmen. He shouldn't be near the Test team, though. I think Badri is a far better fit in the 5-day outfit.