str accessor fails for object-typed data that is actually numeric
One thing that is important for me, since convert_objects has been deprecated (#11221), is that removing it from my code yields a bug that I've reproduced in simplified form here:

``` python
import pandas as pd
import numpy as np
df = pd.DataFrame({'a': [1, 2]})
print df.dtypes  #The dtype of a is an int64
df.a = df.a.astype(np.object)
print df.dtypes  #Now, the dtype of a is an object
df.a.str.decode('utf-8', errors='ignore').str.encode('utf-8')
## AttributeError: Can only use .str accessor with string values, 
## which use np.object_ dtype in pandas
```

In my code, I use convert_objects to convert everything that can be converted away from an object into something else, then I use the column's dtype to check if it can be handled by the unicode decode-encode step.
