Yes, you could do that also.
The random solution, on the other hand, is more complete, since it is safer. It is a real life solution, so it is not mandatory (the increment is enough), but since it is not that difficult to do, I see no problem.
Just imagine if your web server used an incremental session ID instead of a random one. If you just saw your session ID is 1234, you could probably use 1233 and get a valid session, luckly with credit card information or other confidential information already inputed.
Also if you want to increase performance, the random solution allows you to generate id's without synchronization. Off course, having the possiblity of collision (larger or smalled depending on keeping tracks or not).