I blame Dan Abnett.
I was re-reading the Gaunt's Ghosts series of novels (a series that I would highly recommend) and his description of the battles of the Imperial Guard were so fantastic that it made me want to collect an Imperial Guard army (back when they were still the Imperial Guard).
I hit ebay to pick up the beginnings of an army and one of my first purchases was the Gaunt's Ghosts squad and some Tanith troopers. When I started the army, these were no longer available from GW directly, so I probably paid more than I should have for them, but I had to own the models that got me collecting them in the first place. My initial plan was to use them as the command squad for my army; Gaunt as the commander and his unit as the command squad. In games these days they tend to be used as a veterans squad with sniper rifles. I would love to have collected an entire Tanith First and Only army, but my modelling skills are insufficient to kit out an entire guard army with camo cloaks.