In my mind, there's two reasons to go with Xeon on desktop:
1. You want power savings - equally clocked desktop Xeons without onboard graphics often have lower power draw (20% less) because they don't have the onboard graphics that i5/i7 CPUs do. If you're using a discrete graphics card you don't need the onboard graphics of i5/i7.
I would have liked to have put a Xeon 1270 in my latest Shuttle because of the lowered power consumption (and therefore lowered temperature) when I built it, but they weren't readily available in Canada. I used an i7-2600K instead. The list prices are very similar - $328 for a Xeon 1270 vs. $317 for an i7-2600K.
I recently sourced a 1260L for a firewall/phone server. It's a huge step up from an Atom processor for supporting VPN connections, and it runs on 45W and cooling is completely passive. It took almost three months to get, but it's a *great* processor for performance per watt.
2. You want to use ECC RAM. If this is the case, make sure the motherboard you're using actually supports ECC RAM - many do not.
If you don't know whether or not you need ECC RAM, you probably don't.
In reality bill, you're not going to notice the difference between an i7-2600K and a Xeon for daily work. The i7-2600 is a fantastic processor. Don't pay more for a Xeon on a desktop unless you're looking for ECC RAM or lowered power consumption.