3-4 base alignment is not used all the time, actually nickel or as Nagy said "sub" packages are used majority of the time. However, this does change your overall personnel and the players you draft. You need 4 starting linebackers, not 3. You need 1 monster DT up the middle and 2 heavy DE's on the end. In a 4-3 you have lighter guys on the line playing a different technique.
LB's are playing a lot of coverage, in a 4-3 you mostly rely on the front 4 getting to the QB. Corners play a lot more zone coverage and you typically always have a 2 deep safety look. In a 3-4 your corners play more man coverage, bump and run, single high safety. So you see the scheme really can have a domino effect.
Not to mention in a Tampa-2 style, you really need a very specific MLB like Urlacher or brooks from....Tampa lol.
