Once the invalid data are eliminated, are global temperature data ready to use?

Not yet. They have to be adjusted to account for all the changes that happen over time:

• The landscape has changed dramatically in the last 100-150 years. Villages have become cities; roads have been paved; trees have been planted and cut down.
• The observers have changed. Weather watchers have retired and been replaced by people living uphill, downhill, or across town.
• Daily times of observation have changed.
• The technology has changed. Mercury-in-glass thermometers have been replaced by more modern systems. Broken instruments have been replaced by new ones whose readings may not be perfectly consistent.

None of these changes has anything to do with climate, but they all leave a mark on temperature data. Some changes are known to raise temperatures, like urbanization; others are known to lower them, like the change from liquid-in-glass thermometers to maximum/minimum temperature systems. If these biases are not compensated for, we cannot understand how the climate itself is changing.