You will expand your case options if a removeable mobo is not part of the must have's.
1) After it is built, how often do you really need to remove the motherboard?
Not for adding or changing drives/ram/cards.
2) It is easier to do a neat job of cable management if everything is fixed in place.
If you plan on lots of motherboard exchanges, then maybe.
As geofelt pointed out one does not need a removeable motherboard tray to remove components inserted into the motherboard. If one needs to remove the motherboard then one has to unscrew all those mounting screws whether one has a sliding tray or not.
What you may want to consider a case with a tray that has a large rectangular hole which makes it easy to remove the new tower style cpu heatsinks.
I have one case with a removable motherboard tray, I have had no use for the feature. The case is a tank made by Coolermaster, but is does not have good air flow, and only room for noisy 8cm fans.
All my other good cooling cases are Antec 3 Super LanBoys, 2, 300's and 1, 900.
I have had many cases over the last 15 years, the best are ones with open air flow in the front and back with 12cm fans. The fan grille shaped like a honeycomb cuts wind noise on the Antec cases.
My Silverstone TJ07 has a removable motherboard tray, but I NEVER use it since my Heatsink is too tall and I have yet to think of a scenerio where I would need it since it is just as easy to uninstall a motherboard in the computer.