One guy I talked to in private made a cheap stackable line
array using 49 cent buyout 4" midranges and he thinks it's
great for his band when they do small gigs.

The proaudio designs are modular;
http://home.earthlink.net/~ralaudio/i/vant/d.jpg

Stack as many as you need, easy install/tear-down.

You can implement this type of stackable design in your house
if you wanted to. Dali Megaline is a stackable home line array but their modules are larger in size.

For high output concert sound where there is potential for
driver abuse, the common tweeters you see used in the home line
array designs {commercial or DIY} may not be robust enough
or offer the lowest distortion compared to the better planar
tweeters found in certain proaudio arrays.