How you handle communications with a future employer is most definitely considered within the selection process.
There are two parts you need to consider for these requests:
- Acknowledging the request
- Completing the assignment
Don't assume that the only means to acknowledge the request is by completing the assignment.
Instead, reply back within a reasonable period of time and suggest a completion date for the assignment. In addition to acknowledging the request, this buys you time to schedule the test to your convenience. (Good) Employers understand that you can't always immediately respond to an email and that you may have other things going on in your life so you can't take the test right then and there.
You don't want to delay too long in taking the test unless you have a valid reason that you explain to the potential employer. Likewise, you want to make sure you do your best on the test. You won't be given any slack for sloppy performance even if you took it immediately upon request. They are using it to screen you out, so do your best to pass it and move to the next stage of interviewing.