Is "fanny pack" transport an effective means of self defense? No. Do some believe it is better to have the tool available incase it may be needed? Yes. It is a personal decision, just like carrying a firearm in every other lawful place is a personal decision.
As to the original question, it will depend upon the circumstances, the responding law enforcement officers and the DA. IL does have a pretty good statute that defines justifiable use of force -
(720 ILCS 5/Art. 7 heading)
ARTICLE 7. JUSTIFIABLE USE OF FORCE; EXONERATION
(720 ILCS 5/7 1) (from Ch. 38, par. 7 1)
Sec. 7 1. Use of force in defense of person.
(a) A person is justified in the use of force against another when and to the extent that he reasonably believes that such conduct is necessary to defend himself or another against such other's imminent use of unlawful force. However, he is justified in the use of force which is intended or likely to cause death or great bodily harm only if he reasonably believes that such force is necessary to prevent imminent death or great bodily harm to himself or another, or the commission of a forcible felony.
(b - In no case shall any act involving the use of force justified under this Section give rise to any claim or liability brought by or on behalf of any person acting within the definition of "aggressor" set forth in Section 7 4 of this Article, or the estate, spouse, or other family member of such a person, against the person or estate of the person using such justified force, unless the use of force involves willful or wanton misconduct.
(Source: P.A. 93 832, eff. 7 28 04.)
(720 ILCS 5/7 2) (from Ch. 38, par. 7 2)
Sec. 7 2. Use of force in defense of dwelling.
(a) A person is justified in the use of force against another when and to the extent that he reasonably believes that such conduct is necessary to prevent or terminate such other's unlawful entry into or attack upon a dwelling. However, he is justified in the use of force which is intended or likely to cause death or great bodily harm only if:
(1) The entry is made or attempted in a violent,
riotous, or tumultuous manner, and he reasonably believes that such force is necessary to prevent an assault upon, or offer of personal violence to, him or another then in the dwelling, or
(2) He reasonably believes that such force is
necessary to prevent the commission of a felony in the dwelling.
( b - In no case shall any act involving the use of force justified under this Section give rise to any claim or liability brought by or on behalf of any person acting within the definition of "aggressor" set forth in Section 7 4 of this Article, or the estate, spouse, or other family member of such a person, against the person or estate of the person using such justified force, unless the use of force involves willful or wanton misconduct.
(Source: P.A. 93 832, eff. 7 28 04.)
(720 ILCS 5/7 3) (from Ch. 38, par. 7 3)
Sec. 7 3. Use of force in defense of other property.
(a) A person is justified in the use of force against another when and to the extent that he reasonably believes that such conduct is necessary to prevent or terminate such other's trespass on or other tortious or criminal interference with either real property (other than a dwelling) or personal property, lawfully in his possession or in the possession of another who is a member of his immediate family or household or of a person whose property he has a legal duty to protect. However, he is justified in the use of force which is intended or likely to cause death or great bodily harm only if he reasonably believes that such force is necessary to prevent the commission of a forcible felony.
(b - In no case shall any act involving the use of force justified under this Section give rise to any claim or liability brought by or on behalf of any person acting within the definition of "aggressor" set forth in Section 7 4 of this Article, or the estate, spouse, or other family member of such a person, against the person or estate of the person using such justified force, unless the use of force involves willful or wanton misconduct.
(Source: P.A. 93 832, eff. 7 28 04.)
eta - illinoiscarry.com is a fantastic site with a wealth of illinois specific information.
In Illinois, it is legal to carry an unloaded firearm and a loaded magazine on your person so long as they are both in an enclosed case and you have a valid FOID card. It is legal to have the means by which to protect yourself if needed. My question is, what happens when you need it? If a mugger tries his trade on you, what happens legally if you draw your weapon? What happens if you fire it? Remember, I live in Illinois. 
banghead