From a practical standpoint, it could make some sense. After all, it should be cheaper to build one stadium instead of two, right? Except it isn't in this case. The tab for the Quakes' much simpler home is estimated to be one-tenth that of the A's ballpark. Come in that cheap, and the supposed efficiencies gained by consolidation are outweighed by other issues.
To illustrate this, I took my ballpark model and laid down a soccer field on top of it. Below is just the lower level. Capacity of the lower level alone is 17,000+.
As you can see, the field itself is a snug fit from corner to corner. A Mount Davis-like set of temporary seating sections (in yellow) would be used from time to time. As would be expected, those sections would tear up the grass like nobody's business. The worst part? The Quakes' season runs concurrent with the A's, so you'd see this all season long. The seats are necessary because if they weren't there all you'd have is a massive gap all the way to the wall, yet that area is a prime seating area for soccer.
In addition, seats down the baseball right field line are angled back towards the infield instead of along the sideline. Those seats would at best have suboptimal views, at worst have obstructed views. Soccer is unlike baseball in that there's no focal point for most of the game, as there is within the 60'6" between the pitching rubber and the plate. Action can occur anywhere on a soccer pitch, and in the case of a routine corner kick or throw-in, it can be generated from the edges. Soccer stadia are designed for all seats to have complete views of the field, a practice that is immediately violated when a regulation field is placed in a baseball stadium.
The upper deck (above) isn't so compromised, mostly because it's further removed from the action. In the previous image, the temporary yellow seats were there. Use of those seats would preclude the use of baseball bleacher seats. Still, the total stadium capacity would be over 31,000, which at this point is too big for MLS.
Going back to the field, think about the logistical problems. MLS teams typically play twice a week, 30 regular season matches per year plus various "friendlies." That translates to about 17 home dates. Assuming that home dates are bunched together and timed to miss A's homestands, there would still be at least eight switchovers per year. Let's say that somehow the $250,000 cost to do the same job at the Coliseum could be cut in half, it would still cost $1 million per year. Project that out for 25 years and index for inflation. And it would still leave the players from both sports hating the field. Practical? Hardly.
