A plot below was taken from Track By Track comparision of TPC only and TPC+HFT reconstruction for y2014 AuAu200 low luminosity runs (days 84-90) (see http://www.usatlas.bnl.gov/~fisyak/star/TbyT/2014).
The plot is  presenting ratio no. of primary tracks to global ones (with |eta| < 0.5)   versus HFT hit pattern = no. of Pixel hits + 10 *(no. of Ist hits)
for TPC only (bullets) and TPC+HFT (squares) reconstruction for positive (black) and negative (red) tracks.

• TPC only gives the ratio ~ 0.55
• TPC+HFT gives : 
  • No. HFT hit  : ~0.42;
  • Pixel and no Ist hits : ~0.25
  • Pixel and 1 Ist hit : ~0.50
  • Pixel and 2 Ist hit : ~0.25
• The conclusion from this plot is that
  • pixel hits without Ist damage about 50% of primary tracks,
  • the requirement to have one Ist hit shows no damange due to pixle hits
  • 2 Ist hits situation is not clear. I guess that this is statistically unsignificant.