Let's not forget that Lin Dan during his long hiatus was still indirectly promoting the sport in all his activities outside badminton and directly when he competed in the domestic league as well as incidentally participating in Peter Gade's Farewell Match, and all of that mainly just for being who he is as a badminton icon. Furthermore, in 2013 during his long break, he was appointed a Laureus sport ambassador for which he went to Rio de Janeiro,Brazil, to fulfill his duties.
Personally I feel that all professional athletes who have devoted their youth and best years of their lives should be allowed and even encouraged to take a sufficient long break for whatever reasons personal or otherwise , eg, to set up family, do other things in life that they always wanted to do but never found the time, catch up with their families and spend quality time with loved ones, forced injury layoff, resume academic or other pursuits, etc or simply to rekindle their love of the sport or just to recharge themselves. As I've said before, you take a rest so as to travel further. More importantly, their is life outside badminton or any sport for that matter, to make our life more complete and fulfilling. As a former world chess champion ,Anatoly Karpov, including a few others used to say, chess is my life but my life is not only chess. To add, the late world chess champion , Mikhail Botvinnik, after winning the 1948 world title actually took three years off to finish his Doctorate in Engineering, after which he defended his title in 1951 without having to go through the qualification process - by the way, in chess, automatic qualification for the defending champions is the norm till today; in tennis, Kim Clijsters took two whole year off to start a family, giving birth to a child, and then won the US open on a wild card entry; Goran Ivanesivic, who retired and came back to win at Wimbledon as a wild card; and we can cite other examples in other sports.
When Lee Hyun Il retired and come back three or four times, nobody is complaining but even welcomes it. So isn't it great, I'd say it's a blessing for badminton, that Lin Dan decides to make a serious comeback? Unless, of course, you've your agenda for thinking otherwise. For the sake of the sport at large, objectively speaking, it is always a good thing to see any player make a comeback as long as they are capable of contributing to the sport. I trust that's not just my opinion, ask Dr Kang, the ex-BWF president.
To be exact, the OP's thread asks about automatic qualification for defending champions, not for Lin Dan specifically, but all defending/reigning champions in general. Implementing that would definitely simplify and clarify matters and still leave wild cards available for special cases or be used as the tournament organizer or BWF deem fit. Whether we should increase the number of entries is beside the point but worth considering as that would allow much lower-ranked players with outside chances of winning a chance to compete as well ( but , as expected, cost concerns and the resultant lengthening of the tournament will be issues to take into account).
Putting aside our personal preferences or biases, anything that is ultimately for the good of the sport overall should be supported for indeed badminton is larger than any single player however great he/she is. Broadly speaking, I feel that automatic qualification for defending champions in the world championships may even be considered for extension in other big tourneys, say the four Premier SuperSeries and,perhaps, the SSF itself, as they can be regarded as badminton's Grand Slams (anyway, that's another point, good to have but not essential).
Pardon me for being long-winded, some of the points bear repeating, and that's just my humble opinion. Lastly, as an aside, we've heard of the sad state of affairs the WC'14 has fallen into with several last-minute unexpected withdrawals due to retirement (Du Pengyu), injuries (Tantowi, Hendrawan and K Tago),and sickness (Simon Santoso). All the more reason why there should be flexibility and proper rules in place to ensure all the top contenders are included to maintain the highest level of competition at the world championships as befitting the stature of world champions.